Aidan Keena's brace from the spot brought him to five goals already this term and further dented Derry City's title hopes by the Camac.
With Derry starting four players here new to the league this year, they need time, but this was loss four in their first four matches and their only complaint could be the questionable red card given to Carl Winchester early in the second half that rendered the game a contest no more.
Keena's 16th-minute penalty gave Saints the edge after a forgettable first half and, even before his 67th-minute second, the game had fizzled out.
Saints, nine points on the board already, will be pleased. Stephen Kenny will have enjoyed that Joey Anang barely had a save to make after conceding three against Sligo on Friday.
The difference in atmosphere at games on Mondays compared to Fridays was evident here, with the crowd struggling to get going, for all that the Candstripes were the better side in the opening 15 minutes.
Still, they created nothing, and were opened up in a flash for the Keena opener, his fourth of the season.
Keena was key in the build-up as he was clearly fouled by Mark Connolly, who was blessed not to get booked; Keena stayed up and found Melia, who was brought down by Ben Doherty.
Whistler Gavin Colfer, at pains to avoid cards throughout the night, could have sent Doherty off but decided that it was more of a coming together.
Referees can be of the view that a red card that would effectively end the game as a contest is to be avoided if there is an element of doubt but, even so, Doherty was fortunate.
Keena drove the spot-kick home into the left-hand side of the goal.
Chris Forrester's 300th league appearance for Saints on Friday against Sligo was marked with a beautiful goal.
He was obliged to play a deeper role here, reminiscent of his early days at Peterborough United, when then-manager Graham Westley opined: "There's so much Michael Carrick in him it's scary."
That was nearly a decade ago and Forrester was key to Pat's controlling things here, winning the ball back midway through the half and Keena then forced Brian Maher into a good save.
From the corner that resulted, City broke at pace and looked to have enough numbers to take advantage but Michael Duffy was forced wide by the impressive Axel Sojberg and Saints cleared.
Sojberg played in Zack Elbouzedi late in the half but he blazed over from a decent position.
Colfer may have been at pains to keep his cards unseen in the first half but his decision to send off Winchester was hugely controversial.
Saints players responded with anger after the Belfast's native's challenge on Melia but replays suggested a booking may have been sufficient. This looked the end of the game as a contest.
McClelland's drive from outside the box nearly sealed the deal but Keena was able to do just that from the spot on 67 minutes.
A disguised pass typical of Forrester found the league's top scorer, who was clipped by makeshift right-back Adam O'Reilly.
Keena blasted home from the penalty, again driving the ball to Maher's right.
Saint Patrick's Athletic: Joseph Anang; Alex Sjoberg, Tom Grivosti, Joe Redmond, Anto Breslin; Zack Elbouzedi (Al-Amin Kazeem 88), Chris Forrester, Jay McClelland, Jake Mulraney (Simon Power 70); Mason Melia (Conor Carty 88), Aidan Keena (Brandon Kavanagh 78).
Derry City: Brian Maher; Adam O'Reilly, Mark Connolly, Kevin Holt, Ben Doherty; Paul McMullan (Sam Todd 56), Sadou Diallo, Carl Winchester, Michael Duffy (Danny Mullen 73); Gavin Whyte (Ciaron Harkin 56), Patrick Hoban (Seán Patton 65).
Referee: Gavin Colfer (Wicklow).
