Kello has earned a one-year deal after keeping six clean sheets in 24 Scottish Premiership appearances since joining the Buddies in October.
Boss Tommy Craig has been impressed with the professionalism of the 32-year-old former Hearts keeper.
He said in the Daily Record: "Marian had an impressive season right up until the last four or five weeks when he picked up an injury and we couldn't play him.
"Overall, he came here and impressed everyone with his professionalism.
"His talent as a keeper is there for everyone to see. To get him on board for another season is good news for me and the St Mirren fans."
But McGowan has not been offered a new contract after four years with the club.
Craig added: "Now is the time for a parting of the ways.
"Paul has been a great servant to St Mirren for the past four years but he needs a fresh challenge."
As Craig builds for the future, defender Mo Yaqub, 20, has extended his deal for the next campaign, while academy graduates Adam Brown, Declan Hughes and Mark Williams have signed first-team contracts.
Meanwhile, the St Mirren boss says there is nothing new regarding midfielder Kenny McLean, who has been linked to Rangers.
"There has been no further movement. Everything else is as it was," he said.