County struck first 10 minutes from the break when Melvin De Leeuw scored with a glancing header from Graham Carey's free-kick.
Aberdeen mounted the first attack in the second minute when Cammy Smith slipped the ball through to Rooney but County goalkeeper Brown was quick off his line to avert the danger.
The Dons looked up for this one and they created another chance in the fifth minute when Shaleum Logan swung over a ball to the back post for Barry Robson, whose point-blank header was well stopped by Brown.
The County goalkeeper was forced to make another stop from Robson shortly afterwards when the Dons' midfielder drilled in a Rooney cutback.
At the other end Carey swung over a dangerous cross from the left but Mark Reynolds got a touch on the delivery before De Leeuw could pounce.
County thought they had taken the lead in the 21st minute when De Leeuw flicked the ball in for Yoann Arquin, who shrugged off a challenge from Alan Tate and coolly tucked the ball past Jamie Langfield. However, the goal was ruled out for offside.
The home side came close again in the 32nd minute when a De Leeuw effort was blocked on the line by Logan.
But County did take the lead three minutes later when the home side were awarded a free-kick after Andy Considine fouled Erik Cikos. The Dons' defender was booked and when Carey curled in his delivery De Leeuw rose to glance a header into the net.
County were rampant and Arquin was sent clear on goal by De Leeuw - but Langfield raced from his line to block his shot.
Aberdeen made a change at the start of the second of the second half with Joe Shaughnessy replacing Tate.
The visitors looked stronger and after Robson crashed a 30-yard drive off the crossbar a Robson shot at the near post hit Yann Songo'o before going behind for a corner. Robson claimed the County defender had handled but referee Brian Colvin was having none of it.
A Smith header clipped the post and Shaughnessy fired the rebound straight at Brown, before County went on the counter-attack and Carey looked to have been impeded by Considine inside the box - but play was waved on.
It was County's turn to apply the pressure and a poor clearance from Langfield fell to De Leeuw, whose long-range shot was gathered at the second attempt before Arquin could get a touch.
The next goal came at the other end. After a flurry of activity around the County goal which included shots from Jack and Considine, which were saved, Rooney drilled the ball into the corner of the net for his eighth goal in 11 games.
With time running out Carey was through on goal and about to shoot when he was taken down by Considine on the edge of the 18-yard box.
The Aberdeen player was shown a straight red card and Brittain sent his free-kick into the wall and behind.
Deep in injury time Songo'o got on the end of a Brittain free-kick but Langfield made a point-blank save to deny him the winner.
In the day's other match, St Mirren's relegation fight suffered another significant blow as they allowed St Johnstone to escape Paisley with a 1-0 win.
Steven MacLean smashed home the only goal of the game after 38 minutes to hand Danny Lennon's side their seventh defeat in their last eight Scottish Premiership fixtures.
Goalkeeper Marian Kello was also replaced at half-time and replaced by deputy Chris Dillo after limping off with injury as salt was rubbed into the Buddies' wounds.
The Perth Saints - guided from the sidelines by Callum Davidson and Alex Cleland while boss Tommy Wright continues his recovery from gallbladder surgery - have now won four of their last five matches and claimed this win without talisman Stevie May.
Having confirmed a top-six spot for the third season in succession with their weekend win over Hibernian, St Johnstone took no chances with their top-scorer ahead of next month's William Hill Scottish Cup semi-final with Aberdeen.
The 22-goal striker dropped to the bench as he was rested alongside defender Frazer Wright and midfielder Gary McDonald.
Lennon's side, meanwhile, were short on confidence but that did not stop the Buddies boss naming the same starting XI who lost 3-0 to champions-elect Celtic on Saturday.
But morale will have dropped to an all-time low now with just seven games left to rescue an increasingly-alarming situation.
Yet St Mirren started with a decent tempo and they forged the first opening after eight minutes as John McGinn wriggled past Lee Croft before stinging the fingers of Alan Mannus.
A Steven Thompson header from the resulting corner skimmed the bar, but the visitors were almost let in by Marc McAusland's slip as James Dunne fired wide soon after.
But Lennon's men continued to be the side who set the pace, coming close again after 21 minutes when Conor Newton's deflected strike landed on the roof of Mannus' net.
St Mirren are second bottom of the table for a reason, though, and they offered St Johnstone another opening six minutes later as David Wotherspoon cut-back was slotted wide by Michael O'Halloran.
The Perth men were growing in confidence as they found space down St Mirren's left-hand side far too often but Lennon's team thought they had the opener when Darren McGregor met Kenny McLean's corner to the back post only to nod over.
However, it was the visitors who took the lead when Saints striker MacLean wandered free at the back post to volley home Wotherspoon's deep corner seven minutes before the break.
It was goal St Mirren could ill afford to lose. But their desperation got the better of Sean Kelly and Josh Magennis when both swung at fresh air just as a chance for an instant leveller dropped into the visitors' danger zone.
Matters only got worse for Lennon as he was forced into a change at the break as Kello limped off. Frenchman Dillo was sent on in his place, while the Buddies boss also hooked Newton as he hoped to gain some extra width by throwing on Gregg Wylde.
St Johnstone made a change too as Wright came on for Tim Clancy and the defender had to be on his toes early on to boot clear after Paul McGowan brought out a good save from Mannus, while Steven Anderson then came up with an important goal-line block to deny Magennis' header.
Wylde's introduction had made a difference as the St Johnstone back four was stretched wider than they liked but the final ball was too easy for the visitors to defend.
At the other end, the Dillo had to bail out his side as he blocked O'Halloran's near-post strike after the hosts were caught pushing up too far.
It should have been St Mirren who spent the final few minutes pushing for a leveller but again Dillo had to dig them out of a hole twice in the closing stages as his side surrendered another result in meek fashion.
- Sports & Recreation
- Melvin De Leeuw
- Barry Robson
- Adam Rooney
- Jamie Langfield
- St Johnstone
- Danny Lennon
- St Mirren