[1] Mulligan made his first team debut with Dundee in the final game of the 2018–19 season against St Mirren, in which he came on as a substitute.

[5] In October 2020, Mulligan signed a new contract with Dundee keeping him at the club until 2023, and immediately joined Scottish League One side Peterhead on a season-long loan.

[6] Mulligan suffered an ankle injury early on in his club debut against Dundee United, and in November would undergo surgery that would keep him out until the following season.

[11] During his successful 2nd stint with the club, Peterhead teammate and media personality Simon Ferry compared his work-rate to that of Gareth Bale, and described him as "powerful, strong and direct", and as a "Rolls-Royce" footballer.

[24] He would make his international debut as a substitute in a goalless draw against Belgium in Group I of the 2023 UEFA European Under-21 Championship qualification.