His parents were descendants of Mayflower settlers and the Scottish Macleans of Mull.
Saunders Mac Lane was born on August 4, 1909, in Taftville, Connecticut, United States. He was a son of Donald Bradford MacLane, a congregational minister, and Winifred (Saunders) MacLane, a teacher of English, Latin and mathematics. Saunders had two brothers, one of which, Gerald MacLane, became a mathematics professor. Moreover, he had a sister, who died in infancy.