He is best known for voicing the character Bandit Heeler in the Peabody Award-winning[1] animated children's series Bluey and as the frontman of the Brisbane-based rock group Custard.

The band included school teachers Paul Medew on bass guitar and Glen Donald on keyboards, and they later recruited Cathy Atthow on drums.

[5] In 1989, McCormack and Medew formed Custard Gun with Shane Brunn (later in Hugbubble, Vanlustbader) on drums and James Straker (later in Melniks).

[2] Custard existed for about a decade, with a number of different drummers including Gavin Herrenberg, Danny Plant, John Lowry and Glenn Thompson (later of The Go-Betweens).

The band released five studio albums in the 1990s: Buttercup/Bedford, Wahooti Fandango, Wisenheimer, We Have the Technology and Loverama, a number of EPs and singles and a greatest hits compilation entitled Goodbye Cruel World.

[18] In November 2006, under the guidance of JJJ, McCormack assembled a band to perform a concert at the Tivoli in Brisbane as a tribute for the late Grant McLennan of The Go-Betweens.

Since 2009, McCormack has been responsible for many film and television scores including The Tall Man, Rake, Redfern Now, House Husbands, Wild Boys, and Blood Brothers.

[20] Since 2018, McCormack has starred in the critically acclaimed ABC Kids Australian animated television series Bluey, voicing the titular character's father, Bandit Heeler.

[22] The series won a Peabody Award at the 84th ceremony for "its unparalleled ability to seamlessly unite childhood bliss with meaningful life lessons.