Moriarty, the son of dual Fitzroy premiership player Geoff, was a lightly built full forward who became a spectacular success after leaving Essendon Football Club at the end of 1923 and crossing to Fitzroy Football Club. Despite standing only 5'10" (178 cm), and weighing approximately 60 kg, he had the ability to jump over opponents and take strong over-head marks, as well as being a goal-sneak. In each of his first three matches with Fitzroy, Moriarty kicked seven goals en route to a then VFL season record of 82.
Moriarty represented the VFL at the Hobart carnival in 1924 and went on to become a permanent fixture in Victorian sides for most of the next decade. He topped Fitzroy's goal kicking list every year between 1924 and 1933, except for 1930, and was voted the club's best and fairest player in 1927 during an era when an award of this type was not made every season. At the end of his career, Moriarty had booted 672 goals in his 170-game VFL career, at an average of almost four goals per game.