Partnering compatriot Jay Andrijic, he won the 2013 Australian Open boys' doubles tournament as a 17 year old.

He successfully defended the title, winning the boys' doubles championship for the second consecutive year at the 2014 Australian Open, this time alongside partner Lucas Miedler of Austria.

In August 2014, Mousley received a one-year ban, backdated to 30 May 2014, after testing positive for ecstasy in March.

[3][4] At the 2017 Australian Open, Mousley and partner Alex Bolt were granted a wildcard entry into the men's doubles main draw.

Unexpectedly, they won three consecutive matches to reach the quarterfinals of the Grand Slam tournament, before being defeated by Spanish duo Pablo Carreño Busta and Guillermo García López in three sets.