Justin is an American songwriter, singer, and activist. Frequently co-writing with Julia Michaels, he has written songs for artists such as Britney Spears, Gwen Stefani, Linkin Park, Kelly Clarkson, Sara Bareilles, Selena Gomez, Justin Bieber, DNCE, Citizen Queen, Kesha, Imagine Dragons, The Knocks, Fifth Harmony, 5 Seconds of Summer, Ariana Grande, Lady Gaga, Raye, Bea Miller, Demi Lovato, Dua Lipa, Måneskin, and Fall Out Boy.

From 2004 to 2014, Tranter was the lead singer of Semi Precious Weapons, a rock band based in New York. Since February 2017, they have been a board member of GLAAD, an organization that promotes LGBT acceptance in the entertainment and news industries. Justin's songs have garnered over 50 million single sales and 40 billion streams on Spotify and YouTube as of May 2021.