Here is Ben Mitchell in 1850:
1850 Sonoma Co, California
Dwelling 67, Family 68
Mitchell Benj. 32 M Laborer Kentucky
Mitchell Lucinda 34 F Kentucky
Mitchell Elijah 12 M Kentucky
Mitchell T.M. 5 M Missouri
Mitchell Mary A. 2 F California

From an anonymous article published in the Alta California 4 Oct 1859 (as reported by Robert Parmalee "Pioneer Sonoma") "[Of the pioneers who came to Sonoma in 1846], Ben Mitchell has moved back to the 'States' with his family."