In 2015 Ludlow won gold with the American team in the 4 × 800 m relay at the World Relays; the team's winning time, 8:00.62, was a new American record. She placed fourth at the United States championships in 2:00.09, missing qualifying for the World Championships in Beijing by 0.04 seconds. The following week she improved her personal best to 1:58.68 at the Paris Diamond League meeting, where she also placed fourth. After Ajee' Wilson withdrew from the World Championships due to an injury, Ludlow was named as her replacement.