Born near Glasgow, Reid worked a number of jobs as a young adult—including wholesaler, army medic (during the Second Boer War), farmhand, lumberjack, and dentist—and immigrated to Canada in 1903. He involved himself in local politics and joined the recently formed UFA, which nominated him to run in the 1921 provincial election as its candidate in Vermilion. The UFA won the election, and Reid served in several capacities in the cabinets of Premiers Herbert Greenfield and John Edward Brownlee, where he established a reputation for competence and fiscal conservatism. When a sex scandal forced Brownlee from office in 1934, Reid was the caucus' unanimous choice to succeed him as Premier.