On-loan Tottenham striker Keane missed two golden opportunities to score in either half and was upstaged shortly after the break by Chris Maguire, who celebrated his own debut with a fine winner for Killie.
The result left Celtic 10 points behind Rangers in the Clydesdale Bank Premier League, but now having played the same number of games, while also giving their opponents’ survival hopes a massive shot in the arm.
The outcome was eerily similar to Roy Keane’s Celtic debut, which ended in a Scottish Cup shock at Clyde.
Celtic boss Mowbray’s hopes of winning the SPL now look further away than ever, but he insisted he was pleased with what he saw of new boy Keane.
“He gives us a different dimension,” he said. “He’s a high-quality footballer. You saw that tonight in flashes. He’ll be better for the game, I’m sure, and getting to know his new team-mates. We’re trying to put a team together that can be successful long term. There were flashes tonight but it didn’t run for us.”
Mowbray has not given up completely on catching Rangers but, after passing up the chance to cut the gap to seven points, admitted his team need to start making inroads soon.
“We’ve got to play Rangers twice, they are games we’ve obviously got to win,” he said. “But it’s obviously going to be difficult for us. We’ve got to start winning very, very quickly.”
Keane’s first attempt on goal immediately followed after the half hour, the Republic of Ireland captain shooting wide from 15 yards.
Seven minutes before half-time, Keane should have scored but he sidefooted straight at Cameron Bell after bursting through one on one with the Killie keeper. Eight minutes after the break Killie got their shock winner. Mehdi Taouil exchanging passes with Tim Clancy inside the box and squaring for Maguire to finish low beyond Artur Boruc.
Keane volleyed Marc-Antoine Fortune’s cross wide with 25 minutes remaining, though he was adamant the shot was deflected.
As time ebbed away Keane saw a second great opportunity go begging when the outstanding Bell saved with his feet to spoil the Dubliner’s debut as Killie held on for a famous victory.
KILMARNOCK: Bell, Clancy (Kiernan 73), Ford, Pascali, Hay, Hamill, Bryson, Taouil (Sammon 62), Kelly, Maguire (Severin 80), Kyle.
CELTIC: Boruc, Hinkel (Caddis 31), Loovens, Hooiveld (Thompson 54), Naylor (Brown 74), Kamara, Ki, N’Guemo, McGeady, Fortune, Keane.
Referee: M Tumilty (Scotland).