Montgomery was born in Scotland in 1817 and arrived in the Oregon Territory in 1838. In 1840 he was employed by the Hudson's Bay Company as a labourer at Fort Nisqually. Over the course of the next few years he was given responsibility for the care of horses and cattle. In 1848 he was sent to Fort Victoria where he demonstrated the castration of bulls and stud horses for the benefit of local farmhands. Hudson's Bay Company records indicate he deserted in 1849 when he declared his intention to become a U.S. citizen, though Fort Nisqually records indicate he was employed there until 1852 when he left the Company with a substantial debt that became the subject of correspondence for a further six years. Montgomery married twice: first in 1854 to Isabella and second in 1864 to Elizabeth. Both were believed to be aboriginal women.