Despite starting brightly, City fell behind in the 26th minute when Aaron Greene hit a low shot from the left edge of the box that beat the keeper's dive.
Before that, Philip Lowry found space on the edge of the Pat's 18 yard line on 20 minutes. Rather than have a shot, he rolled the ball to the right for Cillian Morrison to hit a first time effort wide of the goal.
Derry keeper Gerard Doherty made a great stop in the 44th minute to deny Killian Brennan what seemed a certain goal when he shot from the edge of the box.
St Pat's went two up on 56 minutes.
James Chambers hit a daisy cutter of a free kick from 25 yards out that went in to the net despite going through a crowd of players from both sides.
Christy Fagan and Greg Bolger could have made a third goal on 73 minutes when they made a two man breakaway that saw poor control in the Derry box let them down.
St Pat's had the class and more importantly the experience to kill off the game and secure the win.
They hit the third goal in the 94th minute through Chris Forrester as he took down a long ball from upfield on the edge of the area and walked by the tired and static home defence to score.
It ended a disappointing night for Derry and sees their poor run go on.
Derry travel to Shamrock Rovers on Monday night for a rearranged league game, due to Rovers European commitment.
Hear Martin Holmes' match report here...
After the game, Martin Holmes got the views of Derry player Josh Daniels.