The left-back, who has played over 120 times for the Millers since joining them in 2015 from Sheffield Wednesday, bagged for just the second time in front of his own fans to seal a vital draw against Reading at the weekend.
Mattock rammed home from close range in injury time to help his side to point against a team around them at the bottom of the table.
It came 11 months after his only previous goal at New York, which was also a last-minute effort, this time to earn an important 1-0 win over Portsmouth in last season's League One promotion push.
The goal against the Royals was especially important as the Millers recovered from a dismal first-half showing that left boss Paul Warne struggling to know which of his 10 outfield players to take off.
In the end it was Richard Wood and Ryan Williams that were the casualties and the home fightback was completed by Mattock's late strike.
"It was a tough first half," Mattock said. "The lads recognised we weren't doing so well and we switched things around in the second half.
"We had to dig in. We did that and ended up with the goal. It's a nice feeling to score. Usually I am at the other end trying to block the goals going in at the last minute."
If Mattock was as prolific in front of goal as he is at collecting cards the Millers would have a player on their hands.
The left-back, who was sent off twice last season, picked up his fifth booking against Reading but the amnesty had passed a few weeks ago.
That means Mattock now has until April to collect 10 before facing a ban.
"It's been a while since I got the last yellow," Mattock said. "The gaffer stayed on my back, saying, '˜Do not get another one because you will be suspended.'
"I have gone past the cut-off so I can get more now!"
Mattock will hope to continue his ever present run in Warne's side when the Millers host his former club West Brom on Saturday.
He played for the Baggies for three years between 2009 and 2012 following a Â£1million-plus move from Leicester.