The 22-year-old has extended his stay at Ibrox until 2024 and will now spend the entire 2022/23 season with the Ayrshire club.
Mayo, who has 11 Scotland Under-21 caps, impressed during a loan spell at Partick Thistle in the Scottish Championship last term, making 36 appearances and helping the Jags notch a record number of clean sheets.
It was the second time he had been on loan at Firhill, with a Covid-blighted stint at Dunfermline in between. He has still to make his competitive debut for the Rangers first-team.
Mayo will now test himself in the top-flight of Scottish football for the first time and becomes Killie’s fourth summer singing.
Manager Derek McInnes believes the club are recruiting a defender on the rise.
He told the club website: “Lewis started really well in his loan spell with Partick last season and showed signs that he’s clearly improving.
“We look forward to working closely with him to develop him and I’m sure he’ll help us in the challenges ahead.”
Meanwhile, forgotten winger Jake Hastie has travelled south to complete a permanent move to League Two side Hartlepool United.
The 23-year-old hasn’t featured for the Rangers first-team since July 2019 and spent the second half of last season on loan at Northern Ireland champions Linfield after failing to establish himself during a stint at Partick Thistle.
Hastie was previously considered as one of the brightest talents in Scotland when he signed for the Light Blues under previous boss Steven Gerrard.
The former Motherwell youth product will finally end his nightmare spell in Govan by linking up with Paul Hartley’s new-look side.
Hastie could be joined by Gers academy team-mate Josh McPake, who has of clubs chasing his signature, including Israeli side Hapoel Be’er Sheva.
However, new Pools manager Hartley, who has already outlined his intention to make moves for players from the SPFL, is hopeful of enticing McPake to England again, having spent time at Harrogate Town, Morcambe and Tranmere Rovers.