Watmore played a key role in the opening goal, his 22nd minute strike saved before cannoning off Dan Jones for an own goal.
And the Notts County full-back endured an evening to forget, bringing down Benji Kimpioka - who had replaced Watmore minutes earlier - for a penalty, Jerome Sinclair scoring.
In the end it was a comfortable victory for Sunderland, sealing their place in the last 16.
Jack Ross made eight changes to the team that drew 1-1 but still named a very strong side to face League Two strugglers Notts County.
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Luke O’Nien, Max Power and Bryan Oviedo the trio to keep their places. Watmore, as expected, led the Sunderland attack alongside Jerome Sinclair.
There was also a welcome return to action for Dylan McGeouch, who had been suffering from a muscular injury in recent weeks.
It was Watmore’s first Sunderland first team game for over a year, the 24-year-old suffered his second ACL injury in a 2-2 draw with Millwall on November 18, 2017.
It has been a long road back for the forward, with Ross keen not to rush the striker back, Watmore has been training with the first team squad for the past six weeks.
Sunderland, lining up in a 3-5-2 system, didn’t get going for a good ten minutes, County enjoying the better early chances.
A Rob Milsom free-kick flew past Robbin Ruiter’s goal before the Dutch keeper was again called upon to gather after Oviedo’s failed to deal with a low cross.
Ten minutes in and Sunderland carved out their first chance, Oviedo slipped the ball through to Watmore who skipped past a defender before laying it off to Sinclair, he passed it first time to Mumba - playing right wing-back - but his effort was blocked.
Sunderland quickly grew into the game and their class began to show. There was acres of space on the left wing for Oviedo and Sinclair to exploit.
The breakthrough came after 22 minutes, McGeouch played a sublime pass through for Watmore to run on to.
His effort from an angle was saved but the deflection cannoned in off Dan Jones. Sunderland, the better side in the first half, kept pressing, Sinclair blasting over on a couple of occasions.
Ruiter was called upon 35 minutes in, he dived low to claw away Kristian Dennis’ header, then got up quickly to smother Will Patching effort.
Five minutes before the break, Watmore was again involved, a nuisance for the Notts County defenders, he drove at goal but his effort from a tight angle was comfortably saved.
Sunderland deservedly led at the break, O’Nien producing a tireless display in the centre of midfield.
HT: Sunderland 1 Notts County 0
The initial plan was for Watmore to play 45 minutes and see how he was at half-time and the forward started the second half.
Oviedo flashed an effort into the side-netting after bursting forward, Watmore was a better option in the middle.
The left-wing back was in acres of space all game.
Watmore showed his class again just before the hour mark, he played a one-two with O’Nien on the edge of the area, delayed his shot, then saw his effort saved by Fitzsimons, an excellent stop.
Watmore was subbed in the 67th minute, Benji Kimpioka on for the final 20 minutes, Watmore given a huge reception as he made his way off the pitch.
Sunderland soon doubled their lead, Kimpioka brought down by Jones and Sinclair stepped up to make it 2-0.
Sinclair forced a good save from Fitzsimons in the 85th minute after drifting past a defender, while Oviedo saw a low volley from a corner routine comfortably saved.
Sub Andy Kellett fired over late on for the visitors but Sunderland held on for the win and to seal their place in the next round.
Mumba saw a late effort saved just before the final whistle was blown.
FT: Sunderland 2 Notts County 0
Sunderland (3-5-2): Ruiter, Bainbridge, Loovens, Ozturk, Oviedo, McGeouch, Mumba, Power ( C ) (Hackett, 83), O'Nien, Watmore (Kimpioka, 67), Sinclair (Diamond, 87)
Subs Not Used:Patterson, Flanagan, Hunter, Maja.
Goals: Jones (OG, 22), Sinclair (pen, 72)
Notts County (4-5-1): Fitzsimons, Hawkridge (Husin, 63), Brisley, Duffy (C), Jones, Patching (Kellett, 79) Hewitt, Milsom, Vaughan, Thomas, Dennis (Stead, 63).
Subs Not Used: Pindroch, Ward, Evina, Campbell.
Booked: Hewitt (29), Duffy (76), Vaughan (87)
Referee: Graham Salisbury.