Substitute Declan O’Brien looked to have rescued a point for the Boynesiders with two goals in a minute, but Patterson had other ideas and produced a classy strike to snatch it for the Candystripes.
The best chance of the opening quarter fell to the visitors on 22 minutes when Patterson beat the offside trap to latch onto a Barry McNamee pass before driving a shot narrowly wide.
Over the next five minutes, though, two even better opportunities presented themselves to Drogheda as Alan Byrne rifled wide from close range and then Cathal Brady also missed the target with the goal gaping.
Not for the first time this season the home side were made to pay for their poor finishing on 39 minutes when, from the third in a series of Derry corners McNamee’s kick was turned into his own net by Alan Byrne.
The Boynesiders almost levelled from another corner as Byrne’s header was nodded off the line by Barry Molloy and Cassidy’s follow-up shot was blocked by teammate Ryan Brennan.
Drogheda were pressing hard for an equaliser when disaster struck on 62 minutes as Philip Hand’s suicidal pass in midfield gifted an opening to Patterson and he duly rolled the ball past Schlingermann.
For a spell it seemed that Derry would canter to victory, but on 77 minutes Drogs sub Declan O’Brien diverted Gavin Brennan’s corner to the net and barely a minute later ‘Fabio’ blasted home to make it 2-2.
But Drogheda’s porous defence came back to haunt them and Patterson was put through to find the top corner with a classy strike.
DROGHEDA UNITED: Schlingermann; Gannon (Grimes, 33), Byrne, Prendergast, Hand; Brady (O’Brien, 52), Cassidy (Foley, 67), O’Connor, R Brennan, G Brennan; O’Neill.
DERRY CITY: Doherty; Madden, S McEleney, McBride, Houston; Deery (Duffy, 72), Higgins, Molloy, McNamee; Griffin, Patterson.
Referee: Alan Kelly (Cork).