In 711 minor league career games, McMillan batted .252 with 591 hits, 74 doubles, 24 triples, and 10 home runs.

He attended Jacksonville University from 1970 to 1973, where he played college baseball for the Dolphins under head coach Barry Myers.

[1] In the winter of 1972, McMillan played with the United States national baseball team during the Amateur World Series in Managua, Nicaragua.

In 23 games played with the Oaks, McMillan batted .207 with 13 runs scored, 17 hits, one triple, one RBI, and one stolen base.

During the 1976 Major League Baseball expansion draft, McMillan was selected by the Seattle Mariners.

[7] With the Red Wings, McMillan batted .274 with 12 runs scored, 26 hits, three doubles, two triples, and 12 RBIs in 39 games played.

In 24 games with New Orleans, McMillan batted .260 with seven runs scored, 13 hits, three doubles, one triple, and two RBIs.

[8] He made his debut in Major League Baseball (MLB) on September 17, against the Kansas City Royals at Kaufman Stadium.

Defensively during his two games with the Mariners, McMillan played shortstop, committing no errors, while making one assist and two putouts.

In 1978, McMillan played the entire season in the minor leagues with the San Jose Missions, who were the Seattle Mariners Triple-A affiliate at the time.

[11] McMillan's final season in professional baseball came in 1979, as a member of the Pirates' Double-A affiliate, the Buffalo Bisons.