Conan Byrne scored on his 100th appearance for St Pat's as the Inchicore side came from 2-0 down with ten men to earn a point in an action-packed clash at the City Calling Stadium.
Longford raced into a two-goal lead with goals from Kevin O'Connor and a Mark Salmon penalty.
St Pat's hit back through Chris Forrester before they had Ian Bermingham sent off.
The 10 men battled back in the second half however with Byrne heading the equaliser on 66 minutes.
With Pat Flynn out suspended, Longford made one change in defence with Jamie Mulhall coming in at right-back.
Club skipper Salmon returned to start in midfield in Tony Cousins' only other switch as Town searched for a hat-trick of wins.
St Patrick's skipper Ger O'Brien made his first appearance of the season following a hamstring injury while last season's topscorer Christy Fagan returned to lead their attack.
Longford didn't lack in confidence on the back of two wins and they started positively to punish a defensive blunder in taking a seventh minute lead.
St Pat's dallied at the back with central defender Kenny Browne misplacing a pass straight to O'Connor.
And the Town midfielder needed little invitation, chipping Saints' keeper Conor O'Malley superbly from all of 25 yards for a delightful goal.
It got worse for the visitors as they were 2-0 down ten minutes later.
In attempting to use his pace to nip past Bermingham inside the area, Town winger Don Cowan was tripped by the Saints' left-back.
Referee Neil Doyle showed no hesitation in pointing to the spot and Salmon planted the resulting penalty into the bottom corner.
Saints' winger Aaron Greene brought a fine save out Paul Hunt before a terrific team goal brought the perfect response from Liam Buckley's side on 23 minutes.
Greg Bolger, Fagan and Byrne worked the ball through for the run of Forrester who calmly dinked it over the advancing Hunt.
And they were almost level two minutes later, but Greene saw his lob inch over Hunt's crossbar.
The visitors revival was knocked back on its heels within a further minute, though, when Bermingham was sent off when he received a second yellow card for another rash foul on Cowan.
Cowan continued to torment Saints, crossing sublimely for David O'Sullivan to head over when he should have extended Town's lead on 37 minutes.
Despite their numerical disadvantage, St Pat's started the second half on the front foot with Browne working Hunt with a header four minutes in.
Fagan then skipped past a couple of defenders to bring a good save with his feet from Hunt.
St. Pat's were stretched on the counter on 55 minutes when Stephen Rice put O'Sullivan through. The striker's weak shot was comfortable for O'Malley, though.
The Saints keeper then had to make a good stop from O'Sullivan after Gary Shaw's flick put him in on goal on 62 minutes.
But St Patrick's continued to press and got their reward with another excellent goal four minutes later as Fagan outfoxed Mulhall on the left to cross for Byrne to head home at the back post to cap his century.
The drama continued unabated until the finish with Longford twice coming close to winning it in stoppage time.
First Kalem Simon had his shot deflected to loop over the bar on 91 minutes.
Fellow substitute Ayman Ben Mohamed then rattled the St Pat's crossbar from 30 yards four minutes into added time, seconds before the final whistle was blown on a cracking game.