The Thomas family traveled to Utah between 1850 and 1853, based on evidence from emigration and census records. According to the obituary of "Margaret Sophie Thomas", they arrived in the year 1851.

William was a return missionary when traveling with the John W. Berry Company arriving in the valley 21 June 1858.

Excerpt from a "Mormon Trail Journal of Thomas Bullock" :

... W[illia]m P. Thomas, age 45, in Church 9 years, Elder, Travelling Elder, England, H.C., residence Box Elder, Farmer

Source: "Mormon Pioneer Overland Travel, 1847–1868"