The Premier League champions were 3-0 up at the break after a dominant first half featuring two Phil Foden goals - one of which he knew very little about.
The visitors started well, Joao Cancelo forcing a smart save and Gabriel Jesus seeing a strike hooked off the line by Lewis Dunk.
City opened the scoring with just 12 minutes on the clock.
Brighton goalkeeper Robert Sanchez spilled the ball under pressure and Ilkay Gundogan made him pay with the finish.
The Seagulls' shouts for a foul went ignored.
The lead was doubled before the half-hour, Bernardo Silva starting a swift counter-attack and feeding Jack Grealish for a low cross towards Foden - who made no mistake.
Two would become three shortly afterwards, Grealish's shot blocked into the path of Jesus whose own effort deflected off Foden and past Sanchez.
He didn't initially know much about it, but Foden was later awarded the goal as replays showed the the bizarre strike was going wide until it hit him.
In the second half, Brighton improved, but could not get the three goals back that they needed.
A late penalty for Alexis Mac Allister proved to be a mere consolation.
Riyad Mahrez responded with a fourth for City in added time, ensuring the game finished 4-1.