McMillan grew up in the heart of North Carolina's basketball country and attended Raleigh's William G. Enloe High School, where he went unnoticed by major college scouts.

McMillan helped lead NC State to a first-place tie in the Atlantic Coast Conference regular season in 1985, and to the Elite Eight in both the 1985 and 1986 NCAA championship tournaments, where the Wolfpack lost to St. John's and Kansas, respectively.

[10] He coached much of the season in a protective boot after surgery and led the team to 50 wins in spite of a historic number of injuries to his key players.

[14] In McMillan's first year as head coach, the team experienced turmoil surrounding the displeasure and eventual departure of All-Star Paul George, who was traded to the Oklahoma City Thunder in June 2017.

[19] The Hawks finished the season 27–11 under McMillan's leadership,[20] ending a four-year playoff drought and earning the fifth seed in the Eastern Conference.

They beat the fourth-seeded New York Knicks in five games, and continued their improbable run by upsetting the top-seeded Philadelphia 76ers in a hard-fought seven-game series.

With that series win, the Hawks made it to the Eastern Conference finals, only the second time in 54 years they have advanced past the second round.

[21] On July 5, 2021, McMillan and the Hawks agreed in principle to drop the "interim" tag from his title and make him the franchise's 14th head coach since the team moved to Atlanta, with a four-year contract.

General manager Travis Schlenk said that while the language of the contract was still being drawn up, "I'm excited he's going to be our head coach going forward.

"[22] The deal was formally announced on July 7, with Schlenk praising the "incredible job" McMillan had done after taking over the team in mid-season.

[26] McMillan was an assistant coach under Mike Krzyzewski for the U.S. national team in the 2006 FIBA World Championship and in the 2008 Beijing Olympics, winning bronze and gold medals, respectively.

[27] He is also a member of the National Junior College Basketball Hall of Fame, due to his All-American performance at Chowan.