The Arsenal loanee, who scored a late equaliser against Tottenham on April 4, went one better to condemn the Hammers – who would have gone third with victory – to a 3-2 defeat at St James’ Park, in the process sending Steve Bruce’s men nine points clear of the relegation zone with just six games to play.
On an eventful afternoon on Tyneside, the visitors trailed 2-0 at the break courtesy of Issa Diop’s own goal and a howler from keeper Lukasz Fabianski which handed Joelinton a second, and had also seen defender Craig Dawson sent off for a second bookable offence.
Remarkably, they dragged themselves back into it through Diop’s header and Jesse Lingard’s 80th-minute penalty – his ninth goal in 10 games – before Willock dashed their hopes in a grandstand finish.
"The players have been fantastic but today we made a couple of mistakes that probably cost us the game," said West Ham boss David Moyes.
"It was costly mistakes that can happen in football. We weren't playing badly - a couple of big mistakes gave Newcastle a leg up. We showed great character in the second half, the sad part was the last 10 minutes.
"Maybe when it went 2-2 they had no choice but to come out and do something else. At 2-2 I was hoping we would be able to see it out. I thought we played well - we made individual mistakes, got a sending off. It wasn't because we played badly."
Newcastle manager Steve Bruce puffed his cheeks out at the final whistle as he toasted a win that pushed his side a giant step closer to Premier League safety.
"I couldn't be more pleased with the way we played first half. We had threat, were dangerous every time, we got men forward," said Bruce.
"All of the stuff in the first half, we didn't do in the second. It is understandable, you go too deep and we made it comfortable for West Ham.
"I had seen the referee running over to have a look and I couldn't understand why. By all accounts it is a penalty. If it is we have to accept it. We tried to change instantly and put two strikers up the pitch. It has paid off.
"It was terrific from Saint-Maximin - it is important that he stays well. He has a badly swollen ankle. He is a very very good player."
"I don't think we are safe yet. With this club there is always a sting in the tail. There is no disputing it has been a difficult last few weeks but we have lost one of our last seven games and are starting to show that bit of consistency."
The defeat for West Ham opens the door for Liverpool and Chelsea to overhaul the Hammers when they play their games in hand.