That year, Macmillan made his screen debut with small roles in the horror film Heartless and the fifth series of the BBC One crime drama Hustle as Harry Fielding.
In 2011, Macmillan starred in the short Friend Request Pending and appeared in the Joe Wright's 2010 action thriller film Hanna.
[7] He starred as Troye King Jones in Simon Amstell's mockumentary film Carnage and began playing Julian in the Channel 4 sitcom Back.
Also in 2018, Macmillan starred as Abs Walter in the Channel 4 sitcom Hang Ups and began appearing in the Black British sketch comedy Famalam.
[8] Macmillan played Laenor Velaryon, Princess Rhaenyra's first husband, in the first season of the HBO fantasy series House of the Dragon, a Game of Thrones prequel and adaptation of George R. R. Martin's companion book Fire and Blood.