Her uncle is Steve Smith, the 1995 Pan American Games silver medalist in the high jump.

[4] At the World Championships she ran 51.48 in the 400m heats to qualify for the semifinals, where she was eliminated running 51.80.

She went on to win a gold medal in the 4 × 400 m relay, along with Jessica Beard, Natasha Hastings and Francena McCorory.

At the Rio Olympics, she won the bronze medal in the 400m hurdles in a personal best time of 53.72 seconds.

In June 2017, Spencer moved into the world all-time top 25 when she improved her 400m hurdles best to 53.11 seconds, finishing fourth at the U.S. Championships, in a race won by Dalilah Muhammad in 52.64.