The 29-year-old has become the club’s 14th summer recruit, signign a deal until 2023, as Tommy Wright rebuilds the squad following relegation to the Championship.
McGowan departed Western Sydney Wanderers on Tuesday having agreed to terminate his contract with one year remaining.
“It’s been a bit of whirlwind having touched down from Australia but I’m delighted to be here and I’m looking forward to helping the club," he told the club’s website.
“I’m looking forward to playing in a new team and for a great club like Kilmarnock so I’m excited to get to started.
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“I think with every move you grow a little bit and I’ve had a career which has taken me to a lot of places that I didn’t think I’d go to but I’ve thoroughly enjoyed it and I’ve learned along the way.
“I’m someone who will always give 100% for the club and I’ve spoken to the gaffer about where he’ll use me in different parts of the game.”
McGowan has played for the Wanderers for the past two seasons and was club captain last campaign.
The Australian moved to Hearts from his homeland in 2008, joining up with brother Ryan who had made the switch two years previous.
He would make 64 appearances before returning to Australia to join Adelaide United. Three impressive seasons saw him return to Europe, joining Portuguese top-flight side Paços de Ferreira.
From there, McGowan had spells with South Korean outfit Gangwon and Vendsyssel of Denmark before joining Western Syndey Wanderers in 2019.
Ryan has recently completed a move to Kuwait SC after departing Sydney FC.