As a freshman at Syracuse in 2014–15, McCullough started in the team's first 16 games of the season before a knee injury suffered against Florida State on January 11 required surgery and ended his campaign.

[11] Four days later, he made his NBA debut, recording 2 points, 2 rebounds, 1 steal and 1 block in just under 11 minutes off the bench in the Nets' 105–104 win over the Denver Nuggets.

[15] On February 22, 2017, McCullough was traded, along with Bojan Bogdanović, to the Washington Wizards in exchange for Andrew Nicholson, Marcus Thornton and a 2017 protected first-round draft pick.

[23] On July 2, 2019, McCullough signed with the San Miguel Beermen as a replacement for Charles Rhodes as the team's import for the 2019 PBA Commissioner's Cup.

[24][25] On his PBA debut, McCullough exploded for a professional then-career-high 47 points to go along with 10 rebounds and 6 3-pointers made to lead the Beermen towards the win against the NLEX Road Warriors.

[31] The Rytas allowed McCullough to return to the team on October 12 as he deeply regretted the incident of violating the club's internal rules and convinced head coach Donaldas Kairys that he is motivated to play.

[34] On June 2, 2021, McCullough was reported to have signed with Al-Ahli of the Bahraini Premier League,[35] he didn't play any games for the team.

[39] He had a brief stint with Philippine team Strong Group Athletics (SGA) at the 2024 William Jones Cup tournament in Taiwan.

A potential return to the PBA in the Philippines was averted with negotiations of Converge FiberXers to sign him as an import for the 2024–25 Commissioner's Cup reportedly failed.

[43][44] McCullough has expressed interest to play for the Philippine national team in 2019, but as a non-Filipino citizen he has to be given citizenship through naturalization to be eligible.

[45][46][47] By 2024, McCullough has lost hope but after his performance at the 2024 William Jones Cup with Strong Group Athletics he has received offers for Taiwanese citizenship to be able to play for the Chinese Taipei national team.