Macdonald was born in Everett, Massachusetts, in 1917 and grew up in Malden. After several years in public school, he entered Phillips Academy in Andover. Macdonald attended Harvard University, where he was captain of the Crimson football team and the roommate of John F. Kennedy. They remained close friends throughout their lives, with Macdonald serving as an usher at then-Senator Kennedy's wedding and as an honorary pallbearer at President Kennedy's funeral. At Harvard, Macdonald earned his B.A. in 1940 and his LL.B. in 1946 from its law school.