He emulated Brando in a performance of his speech from the Joseph L. Mankiewicz film Julius Caesar in an unsuccessful audition and it helped him find his own voice as an actor.

Harris' first lead role was as Callum McGregor in a stage adaptation of the Noughts & Crosses novel series by author Malorie Blackman, OBE.

[6] Harris appears in the Apple TV+ series Ted Lasso as Welsh winger Colin Hughes.

[10][11][12] Co-creator of Ted Lasso, Brendan Hunt, said "Billy's been really amazing in this part" as the series discusses issues such as male vulnerability and homosexuality which are still taboo in the world of association football.

[15][16] His upcoming projects include voice acting in an animated show by writer and previous collaborator, Elliot Blake.