It looked as if United were heading towards a 2-0 defeat at Goodison Park as goals in each half from Everton forwards Moise Kean and Dominic Calvert-Lewin respectively gave the hosts a healthy advantage as the game approached the 94th minute.
However, incredible drama unfolded in stoppage time as United substitute Florian Lejeune netted his first ever goal for the Magpies - an impressive overhead kick - before the French defender sparked chaotic scenes after firing the ball over the line in the 95th minute to grab a point for Steve Bruce's side.
"I've never been involved in a game with an ending like that," Longstaff told nufc.co.uk. "I'm still in a bit of shock. I think that game shows the fight that we have shown throughout this season and the never-say-die attitude we have even when we weren't at our best.
"We can still cause a threat and I think Florian will remember that moment for a while, scoring two goals in the last minute. It was absolutely mental!
"His first goal was an overhead kick and he'll be buzzing with that as well as scoring the equaliser. The substitutions were good for us and helped bring a lot to the team, especially with Flo scoring twice, which is really big for us.
"I was waiting for Emil to try and kick the ball in (for United's second goal) as he had about two or three attempts. I thought (Jordan) Pickford had managed to get a hold of it and we thought it might have been gone but luckily it's counted when we turned to the referee.
"It was a crazy finish when everyone ran straight to the supporters and I imagine they'll be heading home a lot happier than they would have after 89 minutes."
Despite the late heroics for the Magpies, Longstaff admitted Newcastle were below par in terms of their performance against Carlo Ancelotti's Toffees but highlighted the squad's determination and work ethic to fight back from the jaws of defeat.
The Magpies now turn their attention to their Emirates FA Cup clash against League One outfit Oxford United on Saturday and the teenage midfielder believes the squad will be relishing the fourth round tie after picking up five points from their last three Premier League fixtures.
The 19-year-old added: "I think we know as a group when we get certain set-plays, we have got people who can go and head the ball and try to score. I think even the second phases were really key with Isaac (Hayden) and Fabian (Schär) getting ahold of them and that kind of never-say-die attitude and the right mentality has ultimately got us the point today.
"We were not at our best today and I think that was obvious to see but the one thing we try to guarantee is hard work and we did that. Ultimately, we kind of ran around for a bit and not got anywhere near it and then, in added time, we managed to make something of the game and get a point out of it.
"There's a bit of momentum now. We'll prepare for Saturday as a club and, as a player, you want to try and go as far as possible in the competition. I think everybody will be looking forward to Saturday."
