Having netted twice against Bray on Monday, the Donegal man repeated the dose against his former club.
With Finn Harps displaying enthusiasm during the early stages, McLaughlin dampened that on the half-hour.
With both sides testing their opposing goalkeepers, it was Derry who moved into the driving seat following a corner.
Patrick McEleney floated the ball into the danger area before the unmarked McLaughlin guided the ball home from close range.
McLaughlin doubled Derry’s lead nine minutes later when he produced a superb finish after a fluent move by the home side.
It was 3-0 after the break when Rory Patterson lobbed Harps keeper Ciaran Gallagher and within two minutes Barry Molloy headed home Ruaidhri Higgins’ cross for his side’s fourth goal.
DERRY CITY: Doherty; Madden, S McEleney (McCallion 70), McBride, McCaffrey; P McEleney (Farren 63) Higgins, Molloy, McLaughlin; Morrison, Patterson (McDaid 74).
FINN HARPS: Gallagher; Bonnar, Hutton, O’Hagan, Green; Harkin (Merritt 58), McGowan, McBride, Coll; Tracey (Duffy 70), McVeigh.
Referee: P Tuite (Dublin).