[6] Starr began his professional acting career early in the 1990s with a small part in Shortland Street and had guest roles in Xena: Warrior Princess.

In 2001, Starr was cast in Mercy Peak as Todd Van der Velter, a guest role that he had throughout the show's three seasons.

He also received a role as the brother of long running character Waverley Wilson in the soap opera Shortland Street.

Detectives and Street Legal as well as a brief stint in Serial Killers, the comedy series that was penned by Outrageous Fortune co-creator James Griffin.

In 2011, Starr joined the cast of the Australian police drama Rush, playing Senior Sergeant Charlie Lewis.

Playing John Smith, an ex-con who, after 15 years in prison, assumes the identity of Lucas Hood, becoming the new Sheriff of Banshee.

In January 2018, it was announced that Starr was cast as The Homelander in The Boys, Amazon Studios's adaptation of the Garth Ennis and Darick Robertson comic book of the same name.

[16][17] Starr’s performance emerged as a stand-out element of the show and established him as an actor, although he was originally dismissive of the role.

"[20]Carrie Witmer of Uproxx praised Starr by writing, "he creates fear, excitement, and comedy with a nightmarish stare or a menacing smile.

"[21] Hannah Gearan of Screen Rant called Starr "The Best Part of The Boys", stating that he provides a "chilling performance with a measured dialogue delivery that still makes Homelander seem grounded.

"[23] Amhara Chamberlayne of Hollywood Insider ranked Starr's Homelander among all-time great villains, and compared his role to characters like King Joffrey, Stringer Bell, and Gustavo Fring.

The same character appeared as a DLC in the video game Call of Duty: Modern Warfare II (2022) which was voiced by and modeled after Starr.

[33][34] He presented the award for Best Supporting Actress in a Limited or Anthology Series or Movie at the 2024 Emmys, alongside Giancarlo Esposito and Kathy Bates.