Prior to winning Miss District of Columbia USA 2017, she was the first runner-up for Miss District of Columbia USA twice, in 2015 and 2016. In 2017, she was crowned Miss District of Columbia USA 2017 by outgoing titleholder Deshauna Barber. On May 14, 2017, McCullough competed in the Miss USA 2017 pageant in Las Vegas. She went on to win the competition, beating out first runner-up Chhavi Verg of New Jersey and second runner-up Meridith Gould of Minnesota. During her time at Miss USA, McCullough received media attention and praise for deciding to compete with her natural hair, which was seen as support for the natural hair movement. McCullough was both criticized and praised for her conservative question answers, stating that health care was a privilege for the working and not a right, in addition to calling herself an "equalist" as opposed to a feminist. She is the seventh Miss USA titleholder born outside the United States.