He performed in his early years in a number of children's film and television roles, for which he was twice nominated for a Young Artist Award.

In February 2002, he appeared in the film Collateral Damage; in December of that year, he played the main character's son in the romantic comedy Maid in Manhattan.

[6] In 2011, Posey was cast in his breakout role as the lead in the MTV television series Teen Wolf, which is based on the 1985 film.

Posey plays high school student Scott McCall, who is bitten by a werewolf and must keep this fact secret while protecting his loved ones from a host of supernatural threats and beings.

That same year, Posey voiced a character in the pilot of the Disney original cartoon SheZow, which aired on May 4 as a part of the Shorty McShorts' Shorts miniseries.

[11][12] On September 25, 2017, it was announced that Posey would join the third season of the VH1 slasher television series Scream, starring in the role of Shane.

[14] On February 20, 2019, it was announced that Posey would star as Michael Emerson in the CW's television reboot series of The Lost Boys.

[24][25] In the following year, Posey appeared as one of the cast members on MTV's reality TV show The Surreal Life.

[30] In December 2016, Posey was a guest DJ at Emo Nite LA's second anniversary at the Echoplex in Los Angeles.

[31] Posey played guitar and contributed vocals in the pop punk band PVMNTS along with guitarist/bassist/vocalist Freddy Ramirez and drummer Nick Guzman.

[33] The two recruited Makeout drummer Scott Eckel after being introduced by music producer and close friend John Feldmann.

[4][44][45] In an October 2020 interview, Posey disclosed that he had begun smoking marijuana at the age of 12 and had developed an addiction.

[46] In March 2021, Posey stated that he was no longer sober but added that he had learned a great deal from his experience of sobriety.