Castleford battled to a courageous 20-10 success at the Super League champions with victory only confirmed following Greg Eden’s spectacular 90m intercept try in the final seconds.
“We haven't won here for 30 odd years,” said Powell, who leaves for Warrington Wolves at the end of the season.
“I'm made up that we’ve done that in my last game here as Cas coach.
“You never know what might happen with the play-offs…
“But we were just awesome and I am so made up for the players.
“I thought they were so courageous tonight.”
Cas had to defend their line constantly for the last 15 minutes despite Saints being down to 12 men following Tommy Makinson’s red card.
“I thought we defended really tough all the way through,” added Powell.
“We dropped some ball early on but once we got hold of it we were the better team until we had that helter-skelter period at the end.
“I’ve no idea what we were thinking then.
“But an unbelievable piece of play from Greg Eden at the end got us in when we were under so much pressure.”
It was Eden’s second try of the night and he just managed to get to the line to seal victory.
Powell added: “He always looked like he was just going to make it.
“He was tightening up. He’s not played much Greg.
“He’s just coming back from a hamstring injury so it was always going to be difficult.
“He normally goes away for players like that.
“But it’s something he’s class at here (intercepts).
“He’s got such an instinctive ability as a player. He comes up with some errors here and there which are infuriating but then he does some special things and he needs to be applauded for coming up with those types of plays.”
Meanwhile, Powell said the victory was a worthy way of remembering John Kendrew, the lifelong Castleford fan who sadly passed away earlier this week.
He was a first-grade rugby league referee from 1982 to 1994 and had been the secretary of the Castleford Rugby League Players’ Association since 1990 but was perhaps most well known as ‘The Singing Referee.’
Often heard around the local areas in many clubs as an entertainer, regularly performing Neil Diamond songs such as ‘Sweet Caroline’, he also kindly raised thousands of pounds for local charitable causes.
Powell said: “John was a massive Cas fan.
"He refereed me, and he was just a champion of a bloke. I want to put on record from myself and everybody at Castleford Tigers that we will miss him.
“A real champion and such a character, he was called ‘The Singing Ref’ and I just loved him as a bloke.
"As a Castleford fan, the club was a massive part of his life.”