First half goals from Jeremy Doku and Matheus Nunes put the Citizens in control, and they almost added a third midway through the second half when substitute Savinho struck the base of the post.
Watford pulled one back in the final four minutes courtesy of a superb goal from Tom Ince curled into the top corner, setting up a nervy finish for the hosts, but Pep Guardiola's side held on to claim a narrow victory.
Here, Sports Mole provides player ratings from Man City's EFL Cup win over Watford.
Man City player ratings vs. Watford: Doku, Nunes on target as Braithwaite makes solid debut
GOALKEEPER
Stefan Ortega - 6/10
Ortega was a spectator for the majority of the contest and was not given the chance to impress. Came close to keeping a clean sheet but conceded four minutes from time.
DEFENCE
Rico Lewis - 8/10
Lewis operated in a hybrid right-back role that allowed him to drift forward into attacking midfield positions, where he set up Nunes on the edge of the area for City's second goal. As he has been throughout this season, the 19-year-old was lively, energetic, composed in possession and was involved in all of City's positive moves in the final third.
Kyle Walker - 6/10
Following a somewhat underwhelming performance at right-back against Arsenal, Walker predominantly played as a right-sided centre-back against Watford and barely put a foot wrong until he got a bit muddled and failed to clear his lines in the build-up to the Hornets' goal late on.
John Stones - 7/10
A fairly quiet evening for Stones after the heroics of scoring a dramatic equaliser against Arsenal on Sunday. A solid defensive performance and he was able to build up valuable match fitness ahead of a busy fixture schedule.
Kaden Braithwaite - 7/10
At the age of just 16, Braithwaite was handed his senior debut on the left side of defence and he looked calm and composed on the ball throughout. He was taken off with 15 minutes remaining after picking up a minor knock, but he enjoyed a solid debut and can hold his head high.
MIDFIELD
Nico O'Reilly - 7/10
In the absence of injured lynchpin Rodri, O'Reilly had the task of playing at the base of City's midfield. A combative display, but he did not always look in complete control.
Matheus Nunes - 8/10
Nunes celebrated scoring his first goal for the club towards the end of the first half, firing an accurate left-footed shot into the bottom corner. The Portuguese was a physical presence in City's midfield who posed a regular threat in the final third and had a couple of chances to double his tally in the second half.
James McAtee - 7/10
Operating in a No.10 role, McAtee linked up well with the likes of Foden, Grealish and Doku in front of him. He was alert to steal the ball from Watford's goalkeeper in the build-up to City's opening goal, and looked dangerous at times when receiving the ball in the area.
ATTACK
Jeremy Doku - 8/10
Doku was a constant livewire down the right flank in the first 45 minutes and posed a serious threat to Watford's backline, scoring City's opener in the fifth minute with a smart right-footed shot. His final pass was lacking at times after scoring, but he impressed before being taken off at the interval.
Phil Foden- 6/10
Starting his first game of the season, Foden struggled to make the desired impact in the final third operating as a false nine. He had six shots on goal but none of them posed a great threat to Watford's goal.
Jack Grealish - 7/10
Grealish assisted Doku for the opening goal and always had the desire to take on Watford's backline down the left flank. One of City's chief creators and looked a constant threat in the final third.
SUBSTITUTES
Savinho - 8/10
Savinho came on at half time for Doku and, like his Bengian teammate, looked lively on the right. The summer signing from Troyes came close to scoring twice after the hour mark; one sweetly-struck shot from inside the area was headed off the line by Tom Ince, before another from a tight angle smacked the base of the far post.
Jacob Wright - 6/10
Brought on in the 72nd minute to play in midfield, 18-year-old Wright slotted straight into the midfield and sprayed a few nice passes towards the wide areas.
Josko Gvardiol - N/A
Gvardiol was brought on for the final 15 minutes to play on the left side of defence. Nothing too significant from the Croatian.
Watford player ratings vs. Man City: Ince scores stunner as Baah impressed in attack
GOALKEEPER
Jonathan Bond - 6/10
Bond was called into action to make five saves throughout the 90 minutes. He was unable to keep out Doku's opener and was caught out by Nunes's long-range strike, but he made a smart save to deny Grealish at the near post in the second half.
DEFENCE
Festy Ebosele - 6/10
Ebosele looked lively at times when going forward as a right wing-back and put in a shift at the back, making five tackles.
Ryan Porteous - 5/10
Porteous was a fault for the opening goal as his back-pass towards Bond ws under-hit. Recovered reasonably well but produced a few misplaced passes in the second half.
Angelo Ogbonna - 6/10
Ogbonna was handed his debut at the heart of Watford's defence after joining on a free transfer from West Ham United in the summer. The 36-year-old was a physical presence in the penalty box and did well to organise his backline in stages of the match.
James Morris - 5/10
Morris struggled at times to cope with the intricate passing sequences City produced in the final third and did not make too many significant defensive contributions.
Yasser Larouci - 6/10
Larouci, who played at left wing-back, struggled to cope with the pace and trickery of Jeremy Doku in the first half, but he did curl a superb cross into the danger zone for Bayo, who headed wide. The Frenchman was eventually taken off on the 60-minute mark.
MIDFIELD
Imran Louza - 6/10
Louza provided support for Watford's three central defenders as a holding midfielder. Made a total of four tackles and was rarely presented with the opportunities to join in with counter-attacks.
Tom Ince - 8/10
Ince was a standout performer for Watford playing in a central midfield role, boasting a 100% pass completion rate. Before scoring his superb curling shot in the closing stages, he was alert to get back on Watford's goalline and deny Savinho a second-half goal with a brilliant headed clearance.
Ken Sema - 6/10
Wearing the captain's armband, Sema was a physical presence in Watford's midfield, but struggled to put his stamp on proceedings.
ATTACK
Kwadwo Baah - 7/10
Baah was a regular threat for Watford on the counter-attack, using his pace and strength to burst into dangerous positions, and was somewhat unfortunate to see his well-taken goal disallowed in the first half.
Vakoun Bayo - 6/10
Bayo was an isolated figure up front for much of the contest and his best chance in front of goal came towards the end of the first half when he thumped a close-range header a few yards wide - a decent chance that went begging.
SUBSTITUTES
Tom Dele-Bashiru - 6/10
Dele-Bashiru came on in the 60th minute, but the former Man City midfielder was unable to make a significant contribution.
Giorgi Chakvetadze - 7/10
Chakvetadze looked lively and put himself about when he came on for the final half-hour. He was central to a promising counter-attack midway through the second half, before setting up Ince for his goal late on.
Rocco Vata - N/A
Came on with 14 minutes remaining and joined in with some of Watford's counter-attacks in the closing stages.
Mattie Pollock - N/A
Pollock was also brought on with 14 minutes remaining and saw out the contest in Watford's three-man defence.
Pierre Dwomoh - N/A
Dwomoh came on for Louza with just seven minutes remaining.