First-half goals from Nestor Gordillo (8'), Sandro Rodriguez (15') and Pedro Manzi (23') sealed three points for the hosts. William Lalnunfela (37') scored the consolation goal for the visitors.
Akbar Nawas made three changes to the team that outclassed Shillong Lajong 4-2. Roberto Eslava returned to the centre of the defence alongside Gourav Bora. Alexander Jesuraj and Jozef Kaplan were replaced by Sriram and Tarif Akhand.
Khalid Jamil made two changes to the Mohun Bagan lineup that beat Aizawl 2-1. Lalramzauva Khiangte and Omar Elhussieny replaced Amey Ranawade and Mehtab Hossain.
Chennai City were off the blocks quickly and pushed forward looking for goals. Mohun Bagan's lacklustre defending made it easy for the league leaders who jumped to a three-goal lead in 23 minutes.
Nestor Gordillo was afforded far too much space and time on the ball both on the left flank and through the centre of the field. Sandro Rodriguez found the winger centrally, eight minutes after the ball started rolling and the Spaniard lobbed Shilton Paul from 40 yards out to put his side in the lead.
A Mohun Bagan corner gave Chennai their second goal in the 15th minute. The delivery was cleared at the near post and second later, Nestor was seen charging at Bagan's goal at the other end. The forward squared the ball to an unmarked Sandro who beat Shilton with a neat finish from the edge of the box.
A crowd of Bagan players stood and watched as Nestor mazed into the box from the left and put it on a plate for Manzi who slotted home from close range in the 23rd minute.
The visitors pulled one back before the break. Aser Dipanda headed Henry Kisekka's cross from the right flank straight at the Chennai goalkeeper but William Lalnunfela was at the right place at the right time to pounce and slam the loose ball into the net.
Despite reducing the deficit, Mohun Bagan looked lethargic and unwilling to commit to a fight after the restart. Chennai City had Mashoor Thangalakath, who was on the bench, sent off for an altercation during the break but the hosts carried on playing like they have done all season.
The Spanish attacking trio of Chennai ran rings around the Bagan defence at will. Nestor Gordillo was afforded far too much space and time on the ball on the left flank and Pedro Manzi was at his creative best through the centre. Sandro Rodriguez troubled an unusual Bagan backline, with Kingsley Eze slotted to play in midfield by Khalid Jamil.
Things turned from bad to worse for Bagan when Shilton Paul rushed off his line to bring down Pedro Manzi who was one-on-one with the keeper after he latched onto a sublime through-ball from Nestor Gordillo. The custodian was punished with a direct red card.
There were more one-touch passing moves and shots at Bagan's goal but substitute goalie Ricardo Cardozo pulled off a few decent saved to deny the in-form Chennai City attack.