The 25-year-old, who was released by Sheffield Wednesday at the end of last season, became the Millers’ second signing of the summer on Friday when he penned a two-year contract to stay in South Yorkshire.
Mattock played 90 minutes for the Owls in their last-gasp 3-2 win over Steve Evans’ men at New York in March.
Given the hosts were winning until the game entered injury time, the atmosphere was raucous with most Millers fans expecting to win the game.
And that stuck in Mattock’s mind when Evans made his move.
“I thought of that day when the chance came to sign for Rotherham,” he said. “As a footballer, you live to play in matches and atmospheres like that. Yes, I’ll have some more of that.
“It’s a great stadium. Everything about it is right. I couldn’t believe it when I played there last season. It was just so nice, so modern.
“The place erupted when Rotherham scored and the atmosphere was just incredible. I was taken aback at how good it was, to be honest.
“I’m looking forward to that first goal going in at New York Stadium with me wearing a Rotherham shirt, whether I score it or not,” he said. “It will be an unbelievable feeling. I know from experience how passionate the Rotherham fans are.”
Mattock spent three years at Hillsborough and played 30 times last season in a defence that was the fourth stingiest in the Championship.
He felt that warranted a new deal with the Owls, but jumped at the chance to hop across the M1 and stay in the second tier.
“I was a bit surprised when Sheffield Wednesday let me go,” he said. “I’d been part of a good defence and had had a decent season.
“But there were two other left-backs there – Jeremy Helan and Claude Dielna, who’s a centre-half but can play full-back – on two-year contracts, so I sensed what might be coming.
“I was on holiday when my agent called and said Rotherham were interested. I thought straightaway that that would be a great move. It was key for me that they were in the Championship.When I got back, I jumped straight on to it.
“I had a great meeting with Steve Evans and knew where I wanted to be playing next season.”