In 2008, he graduated from Granby High School and went on to attend the BFA Music Theater Program at Elon University in North Carolina for two years.

[5][8][9] On November 8, 2011, he debuted on the television series Glee as Sebastian Smythe, an openly gay member of the Dalton Academy Warblers.

[5] Gustin won the recurring role of Sebastian, a promiscuous and scheming character, after "an exhaustive, weeks-long casting search".

[10] He began filming the role early on Monday, September 26, 2011, after having finished his final West Side Story performance the previous Friday night.

[11] In January 2012, Naya Rivera (as her character Santana Lopez) and Gustin (as Smythe) performed "Smooth Criminal" as a duet in the season three episode "Michael", backed by musical duo 2Cellos.

The project also stars actresses Annabeth Gish and Jessica Lowndes and was shot in West Kelowna, British Columbia, Canada.

[19] As part of the greater Arrowverse, Gustin reprised his role as Allen on Arrow, Supergirl and Legends of Tomorrow, as well as the web series Vixen.