Mainoo signed a long-term deal on Thursday keeping him at Old Trafford until June 2031, replacing a previous contract that had been set to expire in 2027. The Stockport-born midfielder joined United as a six-year-old and has made 98 senior appearances, including scoring the winning goal in the 2024 FA Cup final against Manchester City.
Carrick was speaking ahead of Sunday's Premier League meeting with Liverpool at Old Trafford.
"I've been hugely impressed with Kobbie since I've been here," the United manager said. "I think I've known him for quite some time in different capacities, but I've certainly been aware of him for a long time. He epitomises this football club, you know? Young players coming through the Academy, who live and breathe the club and come through with real talent."
The extension was widely celebrated inside the club after months of uncertainty. Mainoo had reportedly pushed for a loan move to Napoli during January after falling out of favour under former head coach Ruben Amorim, who had restricted him to a single start — in the Carabao Cup at Grimsby — for the entirety of his tenure.
Since Carrick's arrival, Mainoo has started 12 of the 13 available games and has been central to United's push for a top-four finish and a return to the UEFA Champions League.
"He's shown what he's doing. The last couple of games, in particular, he's been fantastic," Carrick added. "The natural thing and the evolution is to sign a new contract and everyone's over the moon with that, including me, because I think he deserves it."
The improvement in form also earned Mainoo a recall to the England squad under Thomas Tuchel, who had not selected him since taking charge given his lack of club minutes under Amorim. His two appearances away from United's first team this season now appear a distant memory.
Carrick said the approach from his coaching staff had simply been to give the 21-year-old freedom and space to rediscover his best.
"Letting him thrive, just giving him that platform to go and be himself has been important. He's evolved, he's improving all the time. You can see that — with more confidence, more responsibility, more belief. He keeps adding to his game."
Director of football Jason Wilcox described Mainoo as one of the most naturally gifted young footballers in the world and said the club had full confidence he would develop into a player capable of competing for the game's biggest honours. Mainoo himself said he wanted to help United fight for major trophies in the years ahead.
Carrick was asked what Mainoo might achieve by 2031, when he will still only be in his mid-twenties.
"Hopefully he's here for a long time and he's part of successful teams that win things, win trophies," he said. "That has to be the aim. It's not going to be easy, it's not going to be straightforward, but that's the challenge."
Sunday's meeting with Liverpool offers Mainoo the chance to make his 99th appearance for the club he has supported all his life, with a win potentially securing Champions League qualification for next season.