The game looked to be heading for a draw until Reid picked up the ball from Dimitri Payet and fired a low shot from the edge of the box into the corner of the net.
Reid is no stranger to scoring dramatic goals and few could forget his winning goal against Manchester United which was the final ever goal at the Boleyn Ground.
He proved to be the hero once again and it was another crucial win for the Hammers.
The victory was the first in the Premier League at London Stadium since August and it was a moment for Slaven Bilic to remember as it was the 100th goal since he took charge of the Club.
Bilic's side came into the game brimming with confidence following their superb victory over Crystal Palace last weekend and showed their fighting qualities once again to clinch all three points right at the death.
The victory was the perfect boost ahead of the big London derby against Chelsea in the EFL Cup on Wednesday night and Bilic will now be hoping to make it three wins in a row and make it a perfect ten-days for the east London Club.
The Hammers manager kept faith with the same system which had done so well to record an impressive victory over Crystal Palace last weekend.
With Aaron Cresswell ruled out through suspension, the Hammers manager handed a first Premier League start to summer signing Edimilson Fernandes.
The Switzerland U21 international almost made an immediate impact after just five minutes when he produced some good work down the left hand side and picked out Dimitri Payet who curled a shot inches past the post.
Payet was involved in the heart of the action once again on ten minutes as he broke free inside the box and saw his shot well kept out by Sunderland keeper Jordan Pickford.
The Hammers were dominating the opening exchanges and were denied by the woodwork on 26 minutes.
The ball fell to Payet who managed to find a bit of space through a mass of defenders and fired a low shot which beat Pickford but the ball cannoned back off the post.
Sunderland finally managed to find a way into the game and created their first major opening on 38 minutes when Patrick van Aanholt found space just outside the area and fired the ball wide of the post.
The danger signs were clearly there for West Ham and Sunderland had another great chance to break the deadlock shortly after half-time when Wahbi Khazri found himself one-on-one with Adrian but the keeper made a comfortable save diving away to his left hand side.
Van Aanholt also had a chance to get his name on the scoresheet on 57 minutes when his shot deflected just past the post.
The home side were also pushing for the crucial opening goal and Fernandes could have made it a dream debut when he was picked out by Payet and struck the ball over the bar.
Bilic brought Jonathan Calleri and Ashley Fletcher into the action as he tried everything to clinch the crucial winner.
And his prayers were answered four minutes into injury time when Reid picked up the ball from Payet and fired a low shot past Pickford into the corner.
Cue the wild celebrations around London Stadium and the fans will now be hoping for more of the same when the Hammers host Chelsea on Wednesday night.
West Ham: Adrian; Reid, Kouyaté, Ogbonna, Antonio (Feghouli 62); Noble, Obiang, Fernandes; Lanzini (Fletcher 83), Zaza (Calleri 68), Payet
Subs not used: Spiegel, Nordtveit, Collins, Oxford
Bookings: Fernandes
Sunderland: Pickford; Manquillo, Koné, O’Shea, Van Aanholt; Watmore (Gooch 74), Rodwell, Ndong, Pienaar (McNair 79), Khazri (Jones 85); Defoe
Subs not used: Domingues, Djilobodji, Anichebe, Honeyman
Bookings: Rodwell, N'Dong, Khazri
Referee: Bobby Madley
Attendance: 56,985