Benoist was born in Houston, but grew up with her mother outside of Denver. She performed in musical theater as a child and after graduating high school, moved to New York City. She graduated from Marymount Manhattan College with a Bachelor of Arts in Theatre Arts in 2011.
After making guest appearances on many TV shows, her breakthrough role came in 2013, playing Marley Rose on Glee. Her next lead role came two years later, playing the title character on Supergirl. In 2018, Benoist made her Broadway debut as Carole King in Beautiful: The Carole King Musical.
Benoist was married to her Glee costar Blake Jenner from 2015 to 2017. She then wed Supergirl costar Chris Wood in September 2019. The pair announced in March 2020 that they are expecting their first child.