He began his acting career in the BBC TV adaptation of I, Claudius (1976), but is best known for portraying Joshamee Gibbs in the Pirates of the Caribbean franchise.

In 1973 he received a scholarship to attend the Royal Academy of Dramatic Art where, in 1975, he won the Best Actor Bancroft Gold Medal.

Along with fellow British actors Ian McNeice and Kenneth Branagh, McNally appeared in two Second World War films, Valkyrie and Conspiracy, that depict behind the scenes activities of high-ranking officials in Nazi Germany.

[9] From 2014 to 2017, McNally played Judge Richard Woodhull of Setauket, Long Island, in the AMC series Turn: Washington's Spies.

In 2019, McNally portrayed Captain Mainwaring in a series of re-enactments of otherwise lost episodes of the sitcom Dad's Army.

[10] In 2021, McNally appeared in three episodes of Flux, Doctor Who's thirteenth series, as Professor Jericho, quickly becoming a fan-favourite character.