Cliftonville remain eight points adrift of Premiership leaders Crusaders after this defeat by Glentoran at the Oval.
It was a scrappy affair in east Belfast with Glens striker Curtis Allen wasting two good chances in the first half.
Jonathon Smith latched on to Stephen McAlorum's flick-on before firing low into the corner for the opener.
James Knowles curled a free-kick over for the Reds before substitute Fra McCaffrey rifled into the bottom corner in stoppage time.
It was a first league defeat for Gerard Lyttle since he was appointed Cliftonville boss and he can have few complaints about the outcome.
His team failed to consistently threaten in the final third although they faced a resolute Glens team which was dangerous on the break.
Allen could have scored twice in the opening 15 minutes, heading over from a Smith cross before firing straight at keeper Conor Devlin.
Stephen Garrett went closest for the visitors in the first half with his header flying over from Martin Donnelly's free-kick.
The impressive Smith broke the deadlock on 55 minutes with a composed finish and McCaffrey added the second as the Reds pressed for an equaliser.
The defeat piles the pressure on Cliftonville ahead of their north Belfast derby showdown against the Crues on Boxing Day.