Born in New York City, he attended Oberlin College in Ohio before transferring to Idaho. MacMillan was the head basketball coach at the University of Idaho. He led the Vandals for seven seasons,from 1920 to 1927, the last six in the Pacific Coast Conference.
In Idaho's first two seasons in the PCC, his upstart program won consecutive conference titles in 1922 and 1923. He also coached baseball and freshman football at Idaho, and baseball at Minnesota from 1942 through 1947.
David was also a longtime head coach at the University of Minnesota (18 seasons, 1927–42, 1945–48), and briefly coached the NBA's Tri-Cities Blackhawks in 1950, succeeding the legendary Red Auerbach.
MacMillan resigned at Minnesota at age 62 in March 1948, citing health reasons. After his brief stint with the Blackhawks, MacMillan served as an assistant coach of the Minneapolis Lakers under John Kundla, who had been a player and assistant under MacMillan at Minnesota.
David also played for the 'New York Original Celtics' in the 1910's.
(The Original Celtics were a barnstorming professional American basketball team. At various times in their existence, the team played in the American Basketball League, the Eastern Basketball League and the Metropolitan Basketball League. The team has no relation to the NBA Boston Celtics, other than as an indirect inspiration. The franchise as a whole was inducted into the Naismith Memorial Basketball Hall of Fame in 1959.)