William is the son of Lovina Monroe Brandon. His father died before his mother crossed the plains with her second husband and younger children in the Roundy company of 1850. William's name is not on the records for the Roundy company with his mother and siblings. First documentation to place him in Utah is a rebaptism in the Utah Stake City Ward on 30 December 1853. It is likely that he arrived that year, but further research is needed to confirm that.
Some sources spell the surname "Munro".
Birth date confirmed by Utah Stake City Ward records at the time of his rebaptism.