He began his career as a child actor, with roles in the comedy drama St. Vincent (2014) and science fiction film Midnight Special (2016).

After playing the title character in the drama The Book of Henry (2017), Martell's breakthrough came with his portrayal of Bill Denbrough in the supernatural horror films It (2017) and It Chapter Two (2019).

His next role that year was as Bill Denbrough in the supernatural horror film It, which premiered to positive critical reviews and grossed over $701 million in the box office, marking Martell's first major commercial success.

He reprised his role as Bill in the 2019 sequel It Chapter Two,[7] marking another commercial success with over $473 million in box office receipts, despite mixed critical reviews.

His final release of 2019, after It, was Rian Johnson's murder mystery comedy film Knives Out, with Martell playing Jacob Thrombey, a white supremacist child.

[8] Also in 2020, it was confirmed by Martell that he was in pre-production for Tunnels, playing the younger siblings of a gun violence victim; however, as of 2024,[update] the film has not yet garnered a release date.

[12] In October 2022, he portrayed Craig in the Netflix film Mr. Harrigan's Phone written and directed by John Lee Hancock, based on the short story of the same name from the If It Bleeds collection by Stephen King.