Saints delivered a fine first-half display before holding off a spirited Tigers fightback to hand stand-in boss Jamahl Lolesi his first win since Keiron Cunningham’s departure.
Matty Fleming, Luke Douglas and Mark Percival all crossed as the hosts took a 20-8 lead into the interval, while Regan Grace’s try with 12 minutes left proved decisive.
Greg Eden and Ben Roberts both touched down twice for Castleford, who suffered only their second defeat of the campaign.
“In the first half we just got beaten for attitude and energy,” Tigers’ head coach Powell.
“We started okay but their speed at the play the ball was far superior to ours.
“We defended well on our line but we made too many errors in posession. Weight of posession at Easter is always going to be crucial – and we were on the wrong end of it.
“We were just not right in between our ears today. I said to the boys at half-time that we needed to keep them out if we were to go on and win - but we didn’t manage to do it.”
Powell also dismissed claims that Grace had fumbled the ball over the line for the killer try.
He added: “The fans thought the try at the end was a knock-on. But it’s a try isn’t it? We have to improve and we will have to respond. The boys are disappointed. We haven’t won here for a long time and we thought we could have done that today.”
Saints caretaker coach Sean Long couldn’t hide his delight at the effort shown by his players just days after the sacking of head coach Keiron Cunningham.
““I’m really pleased with the effort and energy,” said Long.
“The boys turned up and it was a short turnaround following the game against Wigan.
“We found a way to win and Cas are a class team – to beat them is pleasing.”
St Helens: Makinson, Swift, Fleming, Percival, Grace, Fages, Smith, Amor, Lee, Walmsley, Taia, Wilkin, Thompson. Substitutes: Douglas, Walker, Peyroux, Knowles.
Castleford Tigers: Hardaker, Minikin, Webster, Shenton, Eden, Roberts, Gale, Lynch, McShane, Moors, Millington, McMeeken, Massey. Substitutes:Holmes, Milner, Sene-Lefao, Cook.
Referee: Chris Campbell (RFL).