text: In 2015, Mackay took the lead role as Richard Miller in Eugene O'Neill's coming-of-age play, Ah, Wilderness! directed by Natalie Abrahami at Young Vic. In July 2015, MacKay filled the title role of Lewis Aldridge in the BBC's two-part television adaptation of Sadie Jones’ debut novel The Outcast. In February 2016, he portrayed the part of Bill Turcotte in the Hulu production of Stephen King's sci-fi/thriller 11.22.63.
  From 29 March to 14 May 2016, MacKay performed the part of Mick in Harold Pinter's play The Caretaker directed by Matthew Warchus at The Old Vic Theatre in London opposite Timothy Spall and Daniel Mays. In the 2016 film Captain Fantastic, MacKay portrayed Bodevan, eldest son of Ben Cash (Viggo Mortensen). In 2017, he played Jack, the main character in Marrowbone, a psychological horror film directed and written by Sergio G. Sánchez, and also starring Anya Taylor-Joy, Charlie Heaton and Mia Goth. MacKay portrayed Prince Hamlet in Ophelia, a 2018 film retelling the story of Shakespeare's play from the perspective of the young female character Ophelia. The film premiered at the 2018 Sundance Film Festival, and included Daisy Ridley, Naomi Watts, Clive Owen, and Tom Felton in the cast. In the 2018 film Where Hands Touch, MacKay portrayed Lutz, a member of the Hitler Youth in Nazi Germany who falls in love with a biracial girl, portrayed by Amandla Stenberg.