McCullough was born in Naples, Italy to Betty Ann Parker and Artensel E. McCullough Sr. Her mother was a member of the United States Navy for 23 years, and McCullough thus lived in various places such as Sicily, South Korea, Japan, and Hawaii. She was later raised and completed high school in Virginia Beach, Virginia. McCullough attended South Carolina State University, where she graduated with a Bachelor of Science degree in chemistry with a concentration in radiochemistry and served as the school's 75th Miss South Carolina State University. While a student, she was a member of the American Chemical Society, the Health Physics Society, and the American Nuclear Society. She has been inducted into the Golden Key International Honour Society and the National Society of Black Engineers.

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.