His mother, Fiona, died in December 2013 from multiple sclerosis, and a few weeks after her death Lewis was cast as Nibs in the film Pan.

[2][4][5] MacDougall made his film debut in 2015 as Nibs in Pan, directed by Joe Wright, after attending an open audition in Morningside.

[6] In the film, a dark fantasy drama, he plays a boy whose mother is dying of a terminal illness, and who begins communicating with a tree monster seemingly living outside his house.

On 6 October 2016, MacDougall attended the gala screening for A Monster Calls, at the BFI London Film Festival, where he was interviewed about his role.

[8] He then co-starred in the road trip comedy-drama Boundaries, opposite Vera Farmiga and Christopher Plummer, which premiered at South by Southwest in March 2018.