The 40-year-old school teacher has agreed a three-year deal with the Buddies after leaving Northern Irish side Coleraine, who he led to Irish Cup glory last season and a second-place finish in the NIFL Premiership.
Speaking to St Mirren’s website, Kearney said: “It’s been a rollercoaster 72 hours to be honest. It’s all been very surreal.
“I had an interview in the summer that went well and I got a really good feeling about the people in charge at the club.
“I didn’t get the job at that point but I said what’s for you won’t go by you and here I am.
“There were a few things I had to sort out but I feel at this stage it’s the right move for me and I can’t wait to get started.”
Kearney replaces Alan Stubbs, who left the club after three losses in their opening four league matches.
Buddies chief executive Tony Fitzpatrick added: “We are absolutely delighted to have Oran in as our new manager. He is a highly impressive individual and I look forward to working with him.”