City led after just 40 seconds - the fastest ever goal in a Club World Cup final - to make it a match mostly free of tension.
Julián Álvarez followed up fastest to meet a rebound off a post from Nathan Aké's shot.
An own goal in the 27th by Fluminense captain Nino decided the game long before Phil Foden's goal in the 72nd, guiding an Álvarez pass into an open net. Álvarez struck again in the 88th.
It gave City a first Club World Cup title and Europe a 16th in 17 editions of FIFA's competition for continental champions.
Victory made Pep Guardiola the first coach to win the Club World Cup with three different teams. He led Barcelona to titles in 2009 and 2011, then Bayern Munich in 2013 with a team that won the Champions League under outgoing coach Jupp Heynckes.
Guardiola celebrated calmly by walking across to console Fluminense coach Fernando Diniz with a handshake and arm on his shoulder.
At the same time, a melee broke out between players in the Fluminense half of the field with 40-year-old defender Felipe Melo involved and confronted by City captain Kyle Walker. Guardiola was later hugging and smiling with Felipe Melo.
Fluminense started with six players born in the 1980s, and bristled with perceived disrespect when told on Thursday of British media drawing attention to the age of their veteran team. City's oldest player in the starting lineup, 33-year-old Walker, was born in May 1990.
Fluminense's most celebrated player, Marcelo, exited after one hour to warm applause and a handshake on the touchline with City substitute Mateo Kovačić, his former teammate at Real Madrid.
One of Marcelo's first touches in the game was an unwise long pass from defence that let Aké advance in space to shoot.
Álvarez was alone in the goalmouth to stoop and score with his chest.
The bounce of the ball was unkind again to Fluminense when Nino slid in to block Foden's pass across the goalmouth, and the ball looped beyond goalkeeper Fabio's reach.
Foden put the game away in the 72nd minute when he slid to poke in a hard cross across the box from Alvarez.
Alvarez completed his brace in the 88th with a strike from just inside the box, prompting delighted City fans to break into "Blue Moon."
In the third-place game earlier in Jeddah, African champions Al Ahly of Egypt won 4-2 against Urawa, the Asian champions from Japan.
with Reuters
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