A senior figure at 28 and almost halfway through his 13th season in the game, the Millers left-back has sampled the good, the bad and the ugly of professional football.
Friends have been made along the way, memories filed for posterity and future reference – and some former team-mates quickly forgotten, too.
After permanent spells at three big clubs in Leicester City, West Brom and Sheffield Wednesday, Mattock now finds himself amid the more humble surroundings of Rotherham, where a special story is germinating under Paul Warne. And he could not be happier.
The Millers may not be as cash-rich as those aforementioned clubs, but they possess a management and group of like-minded individuals who look at what they have got as opposed to what they have not. And they have plenty.
Talk of spirit and togetherness can be a hackneyed cliché in football, but not where the Millers are concerned. Or, for that matter, today’s derby opponents Sheffield United.
Both sets of players and staff possess those esteemed qualities in abundance, enjoy each other’s company on and off the field and view themselves as good friends – and watch each other’s backs too.
This said, it was not always the way as far as Rotherham are concerned. Certainly not during the Millers’ calamitous relegation campaign of 2016-17 when there were fissures in the dressing room and some players were plainly in it for themselves.
That is when Warne got to work and started to heal the rifts.
On the differences between now and then, Mattock – who has had own recording studio and specialises in “deep house and techno” as a DJ back in his home city of Leicester – said: “That (disunity) is what we have not got this year.
“Everyone is in the same boat and there are no massive egos. If there was the gaffer would ship them out straightaway.
“He likes every player to not only be a good player, but a good person on and off the pitch. He calls them ‘good human beings’ and it is right. Not only that, it makes the football more enjoyable and you go into training with a smile on your face.
“I have been at clubs like Leicester and West Brom who were very successful teams and when I was at West Brom we got promoted to the Premier League.
“But I can always remember the lads kind of arguing. But the togetherness of this side definitely shines through a lot more than what I have seen at any other club. It just seems to work.”
A telling phrase uttered in an article earlier this week by Accrington manager John Coleman, who spoke about investing in the currency of relationships due to an inability to influence people with cash, is likely to have chimed with his Millers counterpart.
Without those monetary advantages while competing in a well-heeled Championship, Warne has instead generated an environment where players and management all sing from the same hymn sheet and learn lessons collectively, enabling the Millers to be rich in another contrasting way.
Mattock added: “Sometimes, I have been at clubs thinking, ‘I do not really want to go in here’ because the egos are that big and they are not my kind of people or ones I would hang around.
“At Rotherham United you go into training and nearly all of the players are my friends and probably will be in the future.
“I put it down to the gaffer and his coaching staff, Richie (Barker) and Hammy (Matt Hamshaw). They have all worked massively hard on bringing this team together and trying to improve us as a team and individuals.
“We have willing lads and a great bunch and a good camaraderie where everyone wants to work for each other and are willing to get better in training every day and the gaffer is mad on that sort of thing.
“We have meetings daily.
“We are doing well and have got a few wins under our belt and are certainly doing a lot better than last time we were in this division.”
Indeed, after 17 games of their last season in the Championship the club had just seven points and were ten points adrift of safety and were about to lose their second manager of a torrid campaign.
Now they have 11 more points and a manager who retains the popularity and full respect of his players, club and supporters.
Work still has to be done and lots of it, but the Millers are in a good place ahead of today’s meeting with the Blades.
Having shaken up the likes of Derby County, Stoke City and Swansea City on home soil, the challenge of making life distinctly uncomfortable for their promotion-aspiring rivals from across the Tinsley Viaduct is likely to sit well with them.
The present-day Millers get in the faces of ‘big’ teams just as they did at their former Millmoor home at this level in the early- to mid-noughties.
On the consistent home form of the 19th-placed Millers, beaten just once on their own patch this season, Mattock said: “I think teams come here and their manager warns them that we are a good home team and opposing players have the thought in the back of their minds that they have got to be up for this.
“When we come out of the traps teams seem to be struggling because we always seem to get the better of teams in the first 10 to 15 minutes and it puts them on the back foot.
“Teams come here knowing that they are in for a game and it is whether the majority of their players are mentally prepared.
“Seven, eight or nine times out of ten they are not that prepared for what we are going to bring. We have just got to keep that up.”
On the Blades, with whom Mattock spent a loan spell in 2010-11, he added: “It will be tough. They are doing well and very attacking-based and on the front foot.
“I played for Portsmouth against Southampton which is a massive game. The hype around derbies is massive and you feel it.
“Fans look forward to it massively and, at the end of the day, football is a game of entertainment and we will do as much as we can to entertain the fans.”