He went on to play Oliver Hide in the BBC medical drama Casualty (2016–2022), Tommy Stubbins in the film Dolittle (2020), and Jacaerys Velaryon in the HBO fantasy series House of the Dragon (2022–present).

[6] The following year, Collett made a guest appearance in the ABC musical comedy series Galavant as a younger version of the titular character (played by Joshua Sasse).

[12] Collett's co-star Durr praised the actor, calling him "absolutely fantastic" and opining, "He's got a great career ahead of him".

[11] In December 2017, it was announced Collett would star as apprentice Tommy Stubbins alongside Robert Downey Jr. in the adventure comedy film Dolittle, which would be released in January 2020.

[15] Collett was then cast as Prince Jacaerys "Jace" Velaryon in the 2022 HBO fantasy series House of the Dragon, a Game of Thrones prequel and adaptation of part of George R. R. Martin's companion book Fire & Blood.