Preston Blair McGuire is listed in the 1860 Census for the Analy Township in the Sonoma County, as P. B. McGuire, a 35 year old Farmer from Kentucky with $800 of real estate and a personal estate of $1000 at dwelling 191. Also living in his household, is who appears to be his wife, Susan, 31 years old and born in Missouri, as well as Wm. P. McGuire a 27 year old farm laborer born in Missouri and who has no real estate but a personal estate of $800. Also listed are a Willard P. McGuire (a. 13, bp. MO), James T. McGuire (a. 12, bp. MO), John J. McGuire (a. 9, bp. IA), Albert W. McGuire (a. 4, bp. CA), and Frances H. McGuire (a. 2, bp. CA). A Cornelius and Susan McGuire along with six children are listed living a few dwellings away at 189 in the same census. Their relationship to Preston's family is not known at this time. D. P. V. Ogan, a school teacher, is also living Cornelius McGuire's household. He later married Francis Leah McReynolds, who is listed living in dwelling 187 with her father, John McReynolds, and step-mother, Martha McReynolds.

There was a John McGuire living in Yuba County who owned a ranch with a Henry Camper in 1851, and in 1854 they sold it to a David Beaver and a Townsend (History of Yuba County, California, p. 91). The connection to our McGuire or the Beaver families, if any, is not known at this time.

Preston appears to have been involved with Allen Harding in seeking a "lost" silver deposit in the Black Rock desert, though the evidence for this is not conclusive.

Cornelia Trosper lists P. B. McGuire's death in her diary as 18 August but does not give the year.