Manaart beat Semmie Onoiafie at Fighting Rookies in May 2016. He won the fight by a second round TKO. He would notch up three more wins in the Dutch circuit, defeating Michael Lorenz, Thomas Bridgewater and
Nico Pereira Horta. Mannaart participated in the 2017
K-1 Heavyweight Grand Prix, being scheduled to fight Masahiro Iwashita in the quarterfinals. He beat Iwashita by a first round knockout, but lost to
Ibrahim El Bouni by knockout in the semifinals. Four months later, Mannaart was scheduled to fight
Antonio Plazibat for the
K-1 Heavyweight title at
K-1: K'FESTA.1. Roel won the fight by unanimous decision. Mannaart subsequently signed with
Glory as well, and fought
Daniel Škvor at
Glory 59: Amsterdam. He beat Skvor by unanimous decision. In his next fight with Glory, Mannaart was scheduled to fight Kirill Kornilov, whom he beat by unanimous decision. Mannaart was scheduled to defend his K-1 title against Chris Bradford, at
K-1 World GP 2019 Yokohamatsuri. He knocked Bradford out in the first round. Roel Mannaart returned to Glory at
Glory 75: Utrecht to fight
Sergej Maslobojev. Maslobojev won the fight by a first round TKO, with Mannaart suffering a broken jaw during the bout. Following the loss to Maslobojev, Mannaart returned to K-1 to defend his Heavyweight title in July of 2025. He would face former K-1 Cruiserweight champion,
K-Jee. Mannaart won the fight via KO in the second round, dropping K-Jee multiple times, before finishing him with low kicks. ==Championships and accomplishments==