He is also known for his appearances in the Marvel Cinematic Universe as Harley Keener in Iron Man 3 (2013) and Avengers: Endgame (2019), as well as the independent film The Whale (2022).
Simpkins also completed Gardens of the Night (2008) and then Revolutionary Road (2008), in which he again starred alongside his real-life sibling Ryan.
In 2013, Simpkins starred alongside Robert Downey Jr. in the live-action film Iron Man 3, as Tony Stark's sidekick Harley Keener (he also reprises the role in Lego Marvel's Avengers).
In 2019, he reprised the role of Harley Keener in a cameo during Stark's funeral at the end of the film Avengers: Endgame.
[12][13] Simpkins played a Christian missionary called Thomas in Darren Aronofsky's Oscar-winning The Whale (2022).