He grew up in Nyack, New York, along with the Mac Arthurs' biological daughter, Mary.
He was educated at Allen-Stevenson School in New York, and later at the Solebury School in New Hope, Pennsylvania, where he starred in basketball, football and baseball.
Deciding to make acting his full-time career, he left Harvard in his second year to make two more Disney movies, Kidnapped and Swiss Family Robinson.
Mac Arthur made his Broadway debut in 1960, playing opposite Jane Fonda in Invitation to a March, for which he received a Theatre World Award.
Mac Arthur grew up around the literary and theatrical talent.