McGregor was born in Amherstburg, Canada West (now Ontario), and was educated at public schools in Windsor but did not attend college.
He subsequently bought a ranch near Medicine Hat (now in Alberta), and owned 800 km2 (200,000 acres) with 10,000 head of cattle and 2,000 horses.
For ten years, McGregor also managed the British-owned Canada Land and Irrigation Company and helped build reservoirs and canal systems near Milo, in Vulcan County, Alberta.
He was the first farmer to cultivate alfalfa in the western provinces, thereby reducing the region's dependency on the wheat market.
He also founded the Glencarnock stock farms of western Canada, and won a number of international livestock prizes.