Bolt pointed to the sky in his trademark "lightning bolt" celebration, but the world's fastest man still had more to offer.
The eight-time Olympic champion was handed a start in Sydney and netted a second-half double, including a neat left-footed finish to open his account, as he staked a claim for a permanent deal.
The travelling side would then make it 3-0 in the 57th minute when the three-time Olympian took the ball wide onto his left foot and blasted a stunning shot past United's keeper.
He capitalised on a defensive mix-up for his second, with a Macarthur defender and goalkeeper colliding to leave Bolt able to tap in from close range.
In his maiden start for the Mariners, the Olympic legend looked nervous early on and, by his own admission, panicked when his first goal-scoring opportunity arrived in the opening minutes.
"I'm here to prove to the world, as I said, that I can be a footballer", Bolt said during a press conference on Friday, according to an ESPN video. "I need more games to get better and to know where to be".
"I think I made some slight mistakes but it's my first real game with the first team".
"His speed endurance is a little bit better but he still looked exhausted after 15-20 minutes".
Whether he signs for the Mariners appears still up in the air, but the brace bolstered his case after a hard first-half where he missed chances and appeared to struggle with the pace and movement of the game.
So therein lies the question - should the Mariners invest in Bolt, or at 32 is the nine-time Olympic gold medal victor too much of a gamble? United plays in a second-tier competition.
The 100m and 200m world record holder has previously trained with Borussia Dortmund, South African club Sundowns and Norway's Stromsgodset.
"I've been improving so I'm just happy I could come out here and show the world I'm improving".
"You can't just react to something that's happened in front of us right now, ' Mulvey said". He played the entire second half against the North Shore Mariners on September 19, when he again alternated between left wing and striker. I've learned a lot. "I've just got to continue learning and getting better".
It's still the preseason for the Mariners - the regular season starts on October 19, and Bolt doesn't yet have a contract.