The Colombian international forward joins from Atletico Nacional, signing a five-year deal with the club.
He will now go on loan to La Liga side Deportivo La Coruna until the end of the 2016/17 season.
Born in Medellini, Moreno was one of the breakout stars in last season’s Copa Liberatadores, South America’s Champions League equivalent, scoring three times in his first three games in the competition which his former side went on to win.
Moreno told ManCity.com: “City are one of the best clubs in the world. It will be nice to experience being part of such a big Club. I want to make the most of it.
“Pep Guardiola is one of the best managers in the world. I think he likes young players very much and he can improve young players tactically and technically. It will be great to work with him and to be part of a great team.”
City Director of Football Txiki Begiristain said: "Marlos is an excellent prospect who has already broken into his national team.
"He's a versatile player, still learning, but we believe he has a fantastic future in the game and with City.
"He makes and scores goals and is an intelligent footballer, we're delighted to have signed him."
It was Moreno’s Copa Libertadores performances which led leading South American football journalist Tim Vickery to declare him “the best young talent on the continent.”
Vickery wrote of Marlos: “There are no guarantees but not since the likes of Sergio Aguero, Alexis Sanchez and Neymar have I been so enthusiastic about a young player.”
Making his debut as a second-half substitute against Rionegro Aguilas as recently as July 2014, Moreno has enjoyed a meteoric ascent, going on to score his first professional goal on 10 September 2015.
An international call-up soon followed in February of the following year and he made his senior bow in March 2015, replacing Juan Cuadrado as an 85th minute substitute, setting up a last-minute winner for Edwin Cardona against Uruguay.
Moreno broke through playing on the left wing where he can cut inside and strike shots with power and precision but he was also noted for his work rate, his intelligent interchanges and ability to see the picture when it comes to passing the ball.
The 19-year old can play anywhere across the front positions and has enjoyed comparisons with Colombian legend Faustino Asprillia.
Moreno becomes City’s seventh signing of summer 2016, joining Leroy Sane, Ilkay Gundogan, Nolito, Oleksandr Zinchenko, Gabriel Jesus and Aaron Mooy at the club.