He set his personal best 78.12 m (256 ft 3 in) using the senior implement on his final throw while winning the 2019 USA Outdoor Track and Field Championships.

At the 2008 World Junior Championships in Athletics, McCullough won a silver medal behind compatriot Walter Henning in a personal best throw of 75.88 m (with a six-kilogram ball).

[4] In 2012 McCullough qualified for the Olympic Trials; there he finished just out of the medals with a throw of 73.55, leaving him in the fourth place position.

[5] His father, Connor McCullagh, competed in the hammer throw for Ireland and was in the 1984 and 1988 Olympics.

[7] At the 2015 World Championships he was the top American, finishing the qualifying round in 13th place, just missing making the final by .20 m (7+3⁄4 in).