Captain Chris Porter gave them the lead on twenty minutes but that was cancelled out by former U's star Ian Henderson's penalty eight minutes later. McDermott then made it 2-1 less than six minutes later, giving the hosts a single goal lead at the break.
A third goal, from Lancashire, just after the hour mark made the game safe, although Porter could have made it interesting but shot wide when well placed.
It was a quiet start to the game, with neither keeper being tested much in the first few minutes.
The U’s did have an effort after six minutes, but Tome Eastman’s attempt to deflect Alex Gilbey’s shot goalwards was unsuccessful and the ball went wide for a goal kick.
Jamie Jones’ first involvement wasn’t too testing, he fell on a weak right wing cross on ten minutes from Michael Lund.
The hosts had a better chance four minutes later, when a long cross was headed back into the danger area by Ian Henderson but Calvin Andrew headed wide from ten yards out when he really should have done better.
At the other end, Chris Porter and George Moncur combined but the U’s captain couldn’t find Moncur with a pass when he could have unleashed a shot himself.
He did unleash a shot on twenty minutes, a shot that put the U’s ahead following another link up between him and Moncur.
Porter was fouled as he tried to go past Lancashire but Moncur had spotted the opportunity to be clear with just the keeper to beat.
His shot was blocked but Porter had been following the play and chested and volleyed home from the edge of the penalty area.
Rochdale had already complained about Alex Gilbey holding Peter Vincenti’s shirt from a corner before the hosts earned another set piece.
More appeals for shirt pulling were ignored by the referee but as the ball went free, Gilbey was harshly adjudged to have tripped Vincenti and a penalty was given.
Former U’s star Ian Henderson was the man tasked with the spot kick and he dispatched his effort into the top corner, giving Jamie Jones no chance.
And five minutes later, Rochdale gained the lead through a sharp finish from Donal McDermott after he had been played in behind the U’s defence.
The two goals in quick succession gave the hosts confidence and they looked comfortable in possession and controlled the majority of the remainder of the half.
Pereira wasn’t tested again before the half time whistle went and the home crowd gave their side a rousing round of applause as they left the field one goal to the good.
McDermott had the first chance of the second half for the hosts, shooting high and wide after cutting onto his right foot just outside the box.
Callum Harriott then repeated things, hitting his effort well over after a positive run from left back Matt Briggs.
Rochdale started to put pressure on the U’s, and a number of whipped in set pieces kept the visitors on the back foot.
The pressure told on sixty three minutes, with Lancashire bundling the ball home at the far post after a free kick was given away wide on the U’s left.
As with the second goal, the home team looked confident after scoring and they moved the ball around without threatening Jones.
Marvin Sordell and Dion Sembie-Ferris were introduced for Edwards and Massey but Sordell only lasted nine minutes before being stretchered off with what looked like a serious knee injury.
He was subsequently replaced by Macauley Bonne as the U’s looked for a way back into the game.
Porter could have made things interesting on seventy two minutes, but pulled his shot wide when well placed.
And then Moncur had a shot deflected just wide, but that was the last effort for the U’s before the whistle went after four minutes of injury time.
With Wayne Brown ruling himself out of the running as permanent manager, a new man is expected to be appointed before the Boxing Day derby against Southend United.