The 22-year-old, who left Glenavon at the end of last season after his contract expired, has agreed a two-year deal with the Bannsiders.
And Kearney is excited by the addition of the Randalstown native, who can play in a number of positions.
"We are delighted to get Jack on board," he said.
"I think for us in particular, there's going to be elements of change this summer and elements of what we want it to look like moving forward.
"Jack is young, fresh, energetic, technically very good and ticks a huge amount of boxes in relation to what we look for in a player.
"He's extremely versatile as well, at international level he's possibly played more as a right-back but at club level he has played all across midfield in a range of different positions.
"It's all of those aspects that attracted us towards him."
O'Mahony made the breakthrough to the first-team at Mourneview Park making 76 appearances for Gary Hamilton's side, netting seven times in the process including one against the Bannsiders last season.
The former Northern Ireland under-21 international will wear the number 21 shirt for the upcoming campaign Coleraine have confirmed.