Murphy James Foster Jr. was born in Franklin, the seat of government of St. Mary Parish. His father, also named Murphy James Foster, was an area sugar planter and owner of oil and natural gas lands, whose own father was Governor Murphy J. Foster Sr. His mother, Olive Roberts (1904-1990), was descended from a prominent family in Minden in Webster Parish and Shreveport in Caddo Parish in northwestern Louisiana. Foster's maternal great-grandfather, Captain Alfred Goodwill (1830-1905), was a native of England and a captain in the Confederate Army. One of the three children of Captain Goodwill and his second wife Ida, was Olive Goodwill (born "circa" 1875). She married Robert Roberts Jr., a former mayor of both Farmerville in Union Parish and Minden, a state representative for Webster Parish, a state district court judge, and an attorney in private practice in Shreveport. Their daughter married Foster's father. One of Foster's cousins, Jasper Goodwill, was the mayor of Minden from 1955 to 1958.