He was employed in a drug shop at Norwich until 1762, when he removed to New Haven and established himself in business as druggist and bookseller. He acquired a considerable property, and engaged in the West India trade, sometimes commanding his own ships, as his father had done. He also carried on trade with Canada, and often visited Quebec. On 22 February 1767, he married Margaret, daughter of Samuel Mansfield. They had three sons, Benedict, Richard, and Henry. She died 19 June 1775. On one of his voyages, being at Honduras, he fought a duel with a British sea-captain who called him a "d*d Yankee"; the captain was wounded and apologized. He occasionally visited England. At noon of 20 April 1775, the news of the battle of Lexington reached New Haven, and Arnold, who was captain of the governor's guards, about 60 in number, assembled them on the College green and offered to lead them to Boston. General Wooster thought he had better wait for regular orders, and the selectmen refused to supply ammunition; but, upon Arnold's threatening to break into the magazine, the selectmen yielded and furnished the ammunition, and the company marched to Cambridge. Arnold immediately proposed the capture of Ticonderoga and Crown Point, and the plan was approved by Dr. Warren, chairman of the committee of safety. Arnold was commissioned as colonel by the Provincial Congress of Massachusetts, and directed to raise 400 men in the western counties and surprise the forts. The same scheme had been entertained in Connecticut, and troops from that colony and from Berkshire, with a number of "Green mountain boys," had already started for the lakes under command of Ethan Allen. On meeting them Arnold claimed the command, but when it was refused he joined the expedition as a volunteer and entered Ticonderoga side by side with Allen. A few days later Arnold captured St. John's. Massachusetts asked Connecticut to put him in command of these posts, but Connecticut preferred Allen. Arnold returned to Cambridge early in July, proposed to Washington the expedition against Quebec by way of the Kennebec and Chaudiere rivers, and was placed in command of 1,100 men and started from Cambridge 11 September. The enterprise, which was as difficult and dangerous as Hannibal's crossing of the Alps, was conducted with consummate ability, but was nearly ruined by the misconduct of Colonel Enos, who deserted and returned to Massachusetts with 200 men and the greater part of the provisions. After frightful hardships, to which 200 more men succumbed, on 13 November the little army climbed the heights of Abraham. As Arnold's force was insufficient to storm the City, and the garrison would not come out to fight, he was obliged to await the arrival of Montgomery, who had just taken Montreal. In the great assault of 31 December in which Montgomery was slain, Arnold received a wound in the leg. For his gallantry he was now made Brigadier-General. He kept up the siege of Quebec until the following April when Wooster arrived and took command. Arnold was put in command of Montreal. The British, being now heavily reinforced, were able to drive the Americans from Canada, and early in June Arnold effected a junction with Gates at Ticonderoga. During the summer he was busily occupied in building a fleet with which to oppose and delay the advance of the British up Lake Champlain. On 11 October he fought a terrible naval battle near Valcour island, in which he was defeated by the overwhelming superiority of the enemy in number of ships and men; but he brought away part of his flotilla and all his surviving troops in safety to Ticonderoga, and his resistance had been so obstinate that it discouraged General Carleton, who retired to Montreal for the winter. This relief of Ticonderoga made it possible to send 3,000 men from the northern army to the aid of Washington, and thus enabled that commander to strike his great blows at Trenton and Princeton.

Among Allen's men concerned in the capture of Ticonderoga in the preceding year was Lieut. John Brown, of Pittsfield, who on that occasion had some difficulty with Arnold. Brown now brought charges against Arnold of malfeasance while in command at Montreal, with reference to exactions of private property for the use of the army. The charges were investigated by the board of war, which pronounced them "cruel and groundless" and entirely exonerated Arnold, and the report was confirmed by Congress. Nevertheless, a party hostile to Arnold had begun to grow up in that body. Gates had already begun to intrigue against Schuyler, and Charles Lee had done his best to ruin Washington. The cabal or faction that afterward took its name from Conway was already forming. Arnold was conspicuous as an intimate friend of Schuyler and Washington, and their enemies began by striking at him. This petty persecution of the commander-in-chief by slighting and insulting his favorite officers was kept up until the last year of the war, and such men as Greene, Morgan, and Stark were almost driven from the service by it. On 19 February 1777, congress appointed five new Major-Generals Stirling, Mifflin, St. Clair, Stephen, and Lincoln thus passing over Arnold, who was the senior brigadier. None of these officers had rendered services at all comparable to his, and, coming as it did so soon after his heroic conduct on Lake Champlain, this action of Congress naturally incensed him. He behaved very well, however, and expressed his willingness to serve under the men lately his juniors, while at the same time he requested Congress to restore him to his relative rank.

The last week in April, 2,000 British troops under Governor Tryon invaded Connecticut and destroyed the military stores at Danbury. They were opposed by Wooster with 600 men, and a skirmish ensued, in which that general was slain. By this time Arnold, who was at New Haven, on a visit to his family, arrived on the scene with several hundred militia, and there was a desperate fight at Ridge-field, in which Arnold had two horses shot from under him. The British were driven to their ships, and narrowly escaped capture. Arnold was now promoted to the rank of Major-General and presented by Congress with a fine horse, but his relative rank was not restored. While he was at Philadelphia inquiring into the reasons for the injustice that had been done him, the country was thrown into consternation by the news of Burgoyne's advance and the fall of Ticonderoga. At Washington's suggestion, Arnold again joined the northern army, and by a brilliant stratagem dispersed the army of St. Leger, which, in cooperation with Burgoyne, was coming down the Mohawk valley, and had laid siege to Fort Stanwix. After Schuyler had been superseded by Gates, Arnold was placed in command of the left wing of the army on Bemis Heights. In the battle of 19 September at Freeman's farm, he frustrated Burgoyne's attempt to turn the American left, and held the enemy at bay till nightfall. If properly reinforced by Gates, he would probably have inflicted a crushing defeat upon Burgoyne. But Gates, who had already begun to dislike him as a friend of Schuyler, was enraged by his criticisms on the battle of Freeman's farm, and sought to wreak his spite by withdrawing from his division some of its best troops. This gave rise to a fierce quarrel. Arnold asked permission to return to Philadelphia, and Gates granted it. But many officers, knowing that a decisive battle was imminent, and feeling no confidence in Gates, entreated Arnold to remain, and he did so. Gates issued no order directly superseding him, but took command of the left wing in person, giving the right wing to Lincoln. At the critical moment of the decisive battle of 7 October Arnold rushed upon the field without orders, and in a series of magnificent charges broke through the British lines and put them to flight. The credit of this great victory, which secured for us the alliance with France, is due chiefly to Arnold, and in a less degree to Morgan. Gates was not on the field, and deserves no credit whatever. Just at the close of the battle Arnold was severely wounded in the leg that had been hurt at Quebec. He was carried on a litter to Albany, and remained there disabled until spring. On 20 January 1778, he received from Congress an antedated commission restoring him to his original seniority in the army. On 19 June as he was still too lame for field service, Washington put him in command of Philadelphia, which the British had just evacuated. The Tory sentiment in that city was strong, and had been strengthened by disgust at the alliance with France, a feeling that Arnold seems to have shared. He soon became engaged to a Tory lady, Margaret, daughter of Edward Shippen, afterward Chief Justice of Pennsylvania. She was celebrated for her beauty, wit, and nobility of character. During the next two years Arnold associated much with the Tories, and his views of public affairs were no doubt influenced by this association. He lived extravagantly, and became involved in debt. He got into quarrels with many persons, especially with Joseph Reed, president of the executive council of the state. These troubles wrought upon him until he made up his mind to resign his commission, obtain a grant of land in central New York, settle it with some of his old soldiers, and end his days in rural seclusion. His request was favorably entertained by the New York legislature, but a long list of charges now brought against him by Reed drove the scheme from his mind. The charges were investigated by a committee of congress, and on all those that affected his integrity he was acquitted. Two charges - first, of having once in a hurry granted a pass in which some due forms were overlooked, and, secondly, of having once used some public wagons, which were standing idle, for saving private property in danger from the enemy - were proved against him; but the committee thought these things too trivial to notice, and recommended an unqualified verdict of acquittal. Arnold then, considering himself vindicated, resigned his command of Philadelphia. But as Reed now represented that further evidence was forthcoming, Congress referred the matter to another committee, which shirked the responsibility through fear of offending Pennsylvania, and handed the affair over to a court-martial. Arnold clamored for a speedy trial, but Reed succeeded in delaying it several months under pretence of collecting evidence. On 26 January 1780, the court-martial rendered its verdict, which agreed in every particular with that of the committee of Congress; but for the two trivial charges proved against Arnold, it was decided that he should receive a reprimand from the commander-in-chief. Washington, who considered Arnold the victim of persecution, couched the reprimand in such terms as to convert it into eulogy, and soon afterward offered Arnold the highest command under himself in the northern army for the next campaign. But Arnold in an evil hour had allowed himself to be persuaded into the course that has blackened his name forever. Three years had elapsed since Saratoga, and the fortunes of the Americans, instead of improving, had grown worse and worse. France had as yet done but little for us, our southern army had been annihilated, our paper money had become worthless, and our credit abroad had hardly begun to exist. Even Washington wrote that "he had almost ceased to hope." The army, clad in rags, half-starved and unpaid, was nearly ripe for the mutiny that broke out a few months later, and desertions to the British lines averaged more than 100 a month. The spirit of desertion now seized upon Arnold, with whom the British commander had for some time tampered through the mediation of John Andre and an American loyalist, Beverley Robinson. Stung by the injustice he had suffered, and influenced by his Tory surroundings, Arnold made up his mind to play a part like that, which General Monk had played in the restoration of Charles II. to the British throne. By putting the British in possession of the Hudson River, he would give them all that they had sought to obtain by the campaigns of 1776-77; and the American cause would thus become so hopeless that an opportunity would be offered for negotiation. Arnold was assured that Lord North would renew the liberal terms already offered in 1778, which conceded everything that the Americans had demanded in 1775. By rendering a cardinal service to the British, he might hope to attain a position of such eminence as to conduct these negotiations, end the war, and restore America to her old allegiance, with her freedom from parliamentary control guaranteed. In order to realize these ambitious dreams, Arnold resorted to the blackest treachery. In July 1780, he sought and obtained command of West Point in order to surrender it to the enemy. When his scheme was detected by the timely capture of Andre, he fled to the British at New York, a disgraced and hated traitor. Instead of getting control of affairs, like General Monk, he had sold himself cheap, receiving a Brigadier-General's place in the British army and a paltry sum of money. In the spring of 1781 he conducted a plundering expedition into Virginia. In September of the same year he was sent to attack New London, in order to divert Washington from his southward march against Cornwallis. In the following winter he went with his wife to London, where he was well received by the king and the Tories, but frowned upon by the Whigs. In 1787 he removed to St. John's, New Brunswick, and entered into mercantile business with his sons Richard and Henry. In 1791 he returned to London and settled there permanently. In 1792 he fought a bloodless duel with the Earl of Landerdale, for a remark, which the latter had made about him in the House of Lords. His last years were embittered by remorse. His life has been written by Sparks in vol. iii. of his "American Biographies," and more fully by Isaac Newton Arnold, " Life of Benedict Arnold, his Patriotism and his Treason" (Chicago, 1880). His fifth son, Sir James Robertson, British soldier, born in Philadelphia in 1780; died in London, England, 27 December 1854. He entered the royal engineers in 1798, and attained the rank of colonel. From 1816 to 1823 he was at the head of the engineers in Nova Scotia and New Brunswick. In 1841 he was transferred from the engineers, and in 1851 was made Lieutenant-General. He served with credit in various parts of the world, displaying especial courage in the attack on Surfnam, where he received a severe wound. He was aide-de-camp to both William IV and Victoria. He bore a strong personal resemblance to his father. Benedict's seventh son, William Fitch, the only one that left issue, born 25 June 1794, was a captain in the British army. His son, Edwin Gladwin, rector of Barrow in Cheshire, inherited the family seat of Little Missenden Abbey, Buckinghamshire, and the grant of land near Toronto, now of great value.