The Millers defender was the unlikely goal hero, smashing home a 90th-minute equaliser in a goalmouth scramble against fellow strugglers Reading.
It was Mattock’s first goal this season – his only strike last season was a 90th-minute winner against Portsmouth – and it was deserved reward for Rotherham’s second-half display.
“Last year I got a last-minute header, I have a knack for them,” said Mattock.
“But it’s a bit strange because I don’t know what to do. I have got no planned celebration so I set off running like a madman.
“I wasn’t sure if I should slide on my knees, do a Klinsmann dive, or what.
“Luckily Joe Newell was there to give me a massive hug, so I got away with it.”
Not that the 28-year-old is prolific. This was only Mattock’s ninth goal in a career that started at Leicester City 11 years ago – covering over 300 games – and has taken in spells with both Sheffield Wednesday and Sheffield United.
He said: “It’s a nice feeling. Usually I am at the other end trying to block the goals going in at the last minute.
“I think we gave it absolutely everything, the whole team, so to be up there at the end I was just watching the ball bounce around – it was flying around. I watched it come to me and instinctively slotted it into the bottom corner.
“It was a nice feeling to get the goal that gave us a draw because it was a tough first half. The lads recognised we weren’t doing so well, and we switched things around in the second half.”
This was a game where you had a crick in your neck from looking left for 90 minutes as all the action came at one over-worked goal.
Reading could have been 3-0 ahead at half-time, and Paul Warne’s harsh words at the interval – “it was like a serial-killer whisper rather than a rant” revealed the Millers’ manager – had the desired effect.
In the second half it was the Rotherham we all know, full of running, not afraid to be direct, and proving a danger from set-pieces. But it looked like being an afternoon of missed chances in the second half until Mattock popped up late on.
Sam Baldock had given the Royals a ninth-minute lead, peeling away behind Richard Wood to drill the ball beyond goalkeeper Marek Rodak.
The striker had already given warning to the Millers when his unmarked header flew over the crossbar.
Rotherham had several half-chances to drag themselves back into the game before the break – those falling to Anthony Forde, Richie Towell and Will Vaulks – but it was the managerless visitors who came closest to a second goal.
John Swift caught the eye with a mazy run, which ended with a shot which failed to match the approach, and the former Chelsea youngster twice was involved in gilt-edged chances being missed.
First Baldock blazed over from eight yards out when it looked easier to score after a perfect low cross from Swift.
Then the striker played a neat one-two with Swift outside the box and the latter saw his shot well saved by Rodak. Zak Vyner did well to chase back and jockey Gareth McCleary, whose subsequent effort was well saved by Rodak.
Rotherham had to improve in the second half, and they did, as Warne sent on Jamie Proctor and Clark Robertson at the break.
“I could have made 10 changes at half-time,” admitted Warne, who saw Anssi Jaakkola twice thwart his side.
The goalkeeper did well to palm away Towell’s fierce effort, blocked Robertson from close-range, and was relieved when Vaulks’s ferocious long-range effort ended up safely in his hands.
Reading were reduced to counter-attacking; Josh Sims hit a post and Rodak was tested by Andy Yiadom’s stinging effort.
Warne revealed afterwards he thought it was going to be one of those days as chances went begging.
Proctor was inches away from Joe Newell’s teasing cross before Yiadom hacked Vaulks’s header off the line from a well-worked corner.
But with the clock ticking down it was a carbon copy set-piece that caused panic in Reading’s goalmouth, and Mattock was on hand to convert from close range.
The result means Rotherham have amazingly drawn 10 of their last 13 games in the Championship.
“We had to dig in, that’s what we did, and ended up with the goal,” said Mattock.
“It was a bad performance in the first half, but a very good performance from the lads in the second.
“We have got some really good positives to take from the last few games. Obviously the wins have not been quite there, but we are not losing either.
“Performances are there and I do feel we have momentum.”