In the 2012–13 NCAA Division I men's basketball season, he was named the Horizon League Player of the Year after leading the Horizon League in scoring at 18.7 points per game and guiding the Detroit Titans to a berth in the 2013 National Invitation Tournament.

He averaged 15.4 points, 4.5 rebounds, 4.0 assists and 1.6 steals per game en route to first team all-conference honors.

On April 27, 2013, McCallum announced that he was leaving Detroit after his junior season and that he was officially declaring himself eligible for the 2013 NBA draft.

[14] On July 9, 2015, McCallum was traded to the San Antonio Spurs in exchange for a 2016 second-round pick.

[15] He made his debut for the Spurs on October 30, recording 2 points and 2 steals in a 102–75 win over the Brooklyn Nets.

[16] During the 2015–16 season, he received multiple assignments to the Austin Spurs, San Antonio's D-League affiliate.

[18] On March 12, 2016, McCallum signed a 10-day contract with the Memphis Grizzlies to help the team deal with numerous injuries.

Memphis had to use an NBA hardship exemption in order to sign him as he made their roster stand at 17, two over the allowed limited of 15.

[19] That night, he made his debut for the Grizzlies in a 95–83 loss to the Atlanta Hawks, recording 13 points, four rebounds, two assists, two steals and one block in 27 minutes off the bench.

On July 26, 2016, McCallum signed with the Detroit Pistons,[22] but was waived on October 24 after appearing in three preseason games.

[23] Six days later, he was acquired by the Grand Rapids Drive of the NBA Development League as an affiliate of the Pistons.

[39] On July 9, 2023, he signed with Victoria Libertas Pesaro of the Italian Lega Basket Serie A (LBA).

[42] On November 8, 2024, McCallum signed with the Taipei Taishin Mars of the Taiwan Professional Basketball League (TPBL).