Sam Baldock netted for the visitors in a one-sided first half, but Mattock snatched a point in a dramatic finish at the AESSEAL New York Stadium.
Rotherham manager Paul Warne was not happy with his side’s first half display.
“It was a disappointing first half and it wasn’t like us,” he said.
“Without Marek (Rodak) we could have been out of the game.
“We had a conversation at half time. It just didn’t feel like it was working.
“I wasn’t happy with anyone in the first half. I don’t like taking people off at half time but we had to change something.
“I could have made 10 changes at half time.
“We needed to get the crowd on our side early on and we did not do that. It made it an edgy game.
“There was a real belief that we were going to get an equalising goal in the second.
“We could not have been as bad in the second half as in the first but I am pleased with a point.
“There was importance to this game because losing to them would have dragged us into a dogfight and would have given them a boost. But there are loads of games to go. If we fall into the bottom three or four we will have to fight our way out.
“The second half was us and in the last 15 minutes it was, apart from the odd Reading counter, us trying to get the goals.”
The Millers were unchanged from the side which drew 2-2 at Sheffield Wednesday last week.
Baldock had the game's first chance, but headed wide for the visitors.
Anthony Forde hammered a volley wide for Rotherham, but Baldock opened the scoring on nine minutes.
He beat the hosts' offside trap and drilled the ball beyond Marek Rodak.
It was a physcial battle against two clubs at the wrong end of the Championship. First Michael Smith dragged his long-range shot wide, then Royals midfielder John Swift failed to test Rodak from distance.
A testing free-kick caused panic in Reading's defence and the loose ball was fired goalbound by Richie Towell only to smash against a wall of bodies.
Will Vaulks was next to try his luck from distance, but his shot zipped the wrong side of the post.
The visitors should have made it 2-0 after two glorious chances.
First, Baldock blazed over from eight yards out, after a perfect low cross from Swift picked him out in the centre.
Then, the striker played a neat one-two with Swift in the box and the latter saw his shot well saved by Rodak.
Rodak kept the Millers in the game with another fine save to deny Gareth McCleary just before half-time.
Rotherham had to improve in the seond half, and they did, as Paul Warne brought on Jamie Proctor and Clark Robertson at the break.
Twice the hosts tested the Reading goalkeeper Anssi Jaakkola, who did well to palm away Towell's fierce effort, before diving low to turn away Robertson's close-range shot.
The Millers were playing with much more urgency, Vaulks hammered his shot straight at the goalkeeper, who also got down to thwart Proctor's effort.
But Reading were dangerous going forward, Rodak fending away Andy Yiadom's stinging effort, while Josh Sims hit the post from close range.
Vaulks blazed over for Rotherham as they pushed forward for an equaliser.
And the hosts will wonder how they did not level, as Proctor was inches away from Joe Newell's teasing cross, before Yiadom hacked Vaulks's header off the line with five minutes left.
But with the clock ticking down, the equaliser finally arrived from an unlikely source, Joe Mattock scrambling the ball home in a crowded box.