After impressing at Manly United, Austin headed to the UK for a stint with Rotherham United, before joining Cambridge after a spell with Stalybridge Celtic.
Austin, 24, also spent time on loan at Lincoln City, Brackley Town and Southport, but returns to Australia for the chance to play in the A-League.
"I'd like to cement a place in the first 11, obviously it's a longer pre-season to what I'm used to so I want to do well in the pre-season and hopefully I'll be in the side and ready to run out," Austin said.
"Over there (in the UK) you are fighting for wins every single week, I'm used to taking a touch and getting smashed so I've gained a lot from that and I'm now quicker on the ball.
"I'm naturally a left-footed player so left-wing is my main position, I'm quite quick, a decent crosser of the ball and hopefully I can chip in with a few goals also.
"Every game (in the A-League) is live in the UK so I've watched quite a bit of it, so I can't wait to have a crack and it's a great opportunity for myself to show what I can do."
Mariners coach Tony Walmsley was pleased to secure the signature of Austin.
"I've known Mitch since being in the UK, one of my jobs at Sheffield was to track young Australians," Walmsley said.
"Now moving back to the Mariners it's important to understand how good young Aussies are playing.
"With the visa restrictions in the A-League and the new model we're putting in place, Mitch is someone that fits that profile; a young Aussie talent with embedded potential that we can expose to the global market.
"Mitch has got incredible pace and great athletic ability, he's six foot one so he immediately contributes some physical requirements we are looking for. He can go down the outside of players, has an excellent cross and gives us genuine width on the left-hand side.
"He's done some real hard yards and that helps shape a young player. The A-League is going to be demanding, but it's going to give him a chance to shine and we believe he can play at a higher level."