Born to Charles Reid and Essex Milliken, at Cardross, Dunbartonshire, Scotland, on 18 April 1811, Reid established a trading house in Hermosillo, Mexico in the late-1820s with a business partner, William Keith, and first visited Los Angeles, then a part of Mexican Alta California, in 1832. He married a "Gabrieleño" woman (a Mission San Gabriel convert renamed Victoria Bartolomea Comicrabit) and adopted her children, María and Felipe.