Blues head coach Frank Lampard gave debuts to defensive duo Reece James and Marc Guehi, as well as a first start to midfielder Billy Gilmour.
Callum Hudson-Odoi also returned after injury to make his first appearance of the new season, while the Mariners made four changes to the side who beat Macclesfield Town 1-0 last time out.
It took just four minutes for the hosts to take the lead, as Ross Barkley powered his way into the Mariners area, before planting a shot past keeper James McKeown.
Three minutes later it was 2-0 to the Blues as Belgian striker Michy Batshuayi took the ball down inside the Mariners box and his left-foot shot left McKeown well beaten.
However, Town responded quickly by pulling a goal back just before the 20-minute mark, with a long ball forward causing problems, allowing Matt Green to net with an superb finish from just outside the area.
The hosts restored their two-goal advantage shortly before the interval as Green was penalised for a foul on Kurt Zouma and captain for the night Pedro slotted home the resulting spot-kick.
Centre-back Zouma got his name on the scoresheet with just over ten minutes of the second half played, as he turned the ball home at close range from a James delivery from a free-kick.
Christian Pulisic saw two shots fly wide as the Mariners were put under further heavy pressure and they conceded a fifth goal eight minutes from time as James fired home a fine effort from just over 20 yards out.
Batshuayi made the score 6-1 four minutes from time with his second of the evening, before England forward Hudson-Odoi marked his return to action with a goal of his own to complete the scoring.
Carabao Cup draw for the fourth round:
Chelsea play Manchester United, Liverpool host Arsena while Colchester travel to Crawley in fourth round of the Carabao Cup. Ties played w/c 28 October
Everton v Watford Aston Villa v Wolves Man City v Southampton Buron v Leciester Crawley v Colchester Chelsea v Man Utd Oxford v Sunderland Liverpool v Arsenal
Chelsea 7-1 Grimsby Town
Blues head coach Frank Lampard gave debuts to defensive duo Reece James and Marc Guehi, as well as a first start to midfielder Billy Gilmour.
Callum Hudson-Odoi also returned after injury to make his first appearance of the new season, while the Mariners made four changes to the side who beat Macclesfield Town 1-0 last time out.
It took just four minutes for the hosts to take the lead, as Ross Barkley powered his way into the Mariners area, before planting a shot past keeper James McKeown.
Three minutes later it was 2-0 to the Blues as Belgian striker Michy Batshuayi took the ball down inside the Mariners box and his left-foot shot left McKeown well beaten.
However, Town responded quickly by pulling a goal back just before the 20-minute mark, with a long ball forward causing problems, allowing Matt Green to net with an superb finish from just outside the area.
The hosts restored their two-goal advantage shortly before the interval as Green was penalised for a foul on Kurt Zouma and captain for the night Pedro slotted home the resulting spot-kick.
Centre-back Zouma got his name on the scoresheet with just over ten minutes of the second half played, as he turned the ball home at close range from a James delivery from a free-kick.
Christian Pulisic saw two shots fly wide as the Mariners were put under further heavy pressure and they conceded a fifth goal eight minutes from time as James fired home a fine effort from just over 20 yards out.
Batshuayi made the score 6-1 four minutes from time with his second of the evening, before England forward Hudson-Odoi marked his return to action with a goal of his own to complete the scoring.
Carabao Cup draw for the fourth round:
Chelsea play Manchester United, Liverpool host Arsena while Colchester travel to Crawley in fourth round of the Carabao Cup.
Ties played w/c 28 October
Everton v Watford
Aston Villa v Wolves
Man City v Southampton
Buron v Leciester
Crawley v Colchester
Chelsea v Man Utd
Oxford v Sunderland
Liverpool v Arsenal