It was a good start to the contest, with decent chances from Dimitri Payet, Jack Rodwell and Simone Zaza in the first 45 minutes but failed to draw first blood.
The same couldn't be said for the second period, with Wahbi Khazri's opportunity looking to be only one of any note, until Reid's late effort securing the three points for Slaven Bilic's men.
The Dimitri Payet show
David Moyes' side were under pressure to secure their first three points as they headed to the London Stadium, but could do nothing as they were dominated by Payet for the opening half hour.
The Frenchman tested the waters early on, as he looked to curl in an effort in the fifth minute, but his effort curled just wide of the mark.
He was at it again in the 11th minute, it was poor defending from the Black Cats, as Payet managed to wriggle through a number of men with no real pressure but his effort was straight at Jordan Pickford.
Payet looked to have secured the lead in the 27th minute, his dummy created a few yards for himself but his low effort came back off the post.
Sunderland didn't get their first look at goal until the 34th minute, as Khazri's free-kick found Rodwell, as he managed to get through the defenders but his header was over the target.
Zaza came close to his first goal in Claret and Blue, as the Italian did brilliantly to try an overhead kick from 15 yards, his effort had Pickford rooted but it was just wide of the mark.
Not following the same beat
Unfortunately for the masses inside the London Stadium, the second period couldn't live up to the first, but there was still one great chance to secure the three points.
It was poor from Hammers in terms of defence, as their failure to clear the through ball allowed Khazri to be clean through on goal, however he fluffed his lines as Adrian saved his effort.
Winning it at the death
It looked like the sides would be sharing the points, as the score remained goalless as they entered stoppage time, but the Hammers managed to secure it right at the death.
Payet quickly played the corner out to Reid on the edge of the area, with the New Zelander's effort going through a host of bodies and Pickford to lift the roof of the London Stadium.