Mike: Thanks for the book reference. I found a few notes on my computer at home, including some of the election data you alluded to. It also appears that he was SF city recorder in 1851. Here's the aforementioned blurb from the Herald:

Among those who leave today for the Atlantic on the steamship Oregon is Thomas P. Johnson, Esq., late sheriff of SF County. He came to this state in 1849. He was appointed by Col. Hays, then sheriff, one of his deputies, in which capacity he served with such ability that, upon the retirement of the colonel, he was named by a unanimous vote of the Court of Sessions, sheriff of the county for the remainder of the term. In this important position he so demeaned himself as to acquire in an extraordinary degree the confidence and esteem of the whole community, and now leaves on a brief visit to his former home in KY, with a reputation which but few men of his age might not justly feel proud. We doubt if any gentleman in SF has a greater number of warm, zealous, and attached friends. [SF Herald, 4-1-54]

It does appear that his "brief visit" home was to get married, or perhaps that was a fortunate consequence of his return. I think it reasonably likely that his son Frener (possibly mistransribed: "Freaner"?) was named for his former colleague and friend.

Johnson was the Know-Thing nominee for SF county sheriff in 1855; I have no further notes but he must have lost. The KN party quickly collapsed, so his political career in CA was likely at an end. He saw the signs and probably decided to go home.