They held off a spirited Burscough side that refused to lie down and accept defeat.
Trinity boss Steve Charles handed summer arrival Gareth Davies his debut after he recovered from injury alongside Danny Anson.
Colin Marrison came back into the side to partner McMahon up front and it was the combination of Marrison and McMahon that won the first penalty on 14 minutes.
The Trinity forwards linked well with some nice touches in midfield, and McMahon fed Marrison through, whose charge towards goal was halted by a tackle only inches inside the box.
The referee had no hesitation in pointing to the spot. McMahon stepped up and sent the keeper the wrong way to give Trinity the lead.
McMahon had a great chance moments later to double the Blues' advantage. Kyle Armstrong failed to deal with Adam Sollitt's long clearance and McMahon was able to get away for a brief second only for Armstrong to recover and make a last ditch tackle to keep the hosts in the game.
Steve Charles was forced into a change midway through the first half when Chris Hall had to leave the field after damaging his ankle. Simon Bird came on in his place, with Marrison dropping into midfield.
Trinity were given a scare when Matty Parry got in behind Neal Spafford and brought the best of Sollitt who produced a magnificent save with his left hand to preserve Trinity's lead.
After the break the host started like a house on fire and had a goal disallowed by the assistant for offside after Dave Roberts turned in a corner from home captain Brian Moogan.
Fortunately the home side seemed to be lacking the cutting edge to get in behind a shaky Blues defence. Burscough did retain the majority of possession, but never forced Sollitt into action.
As the half wore on, the visiting side were able to restrict Burscough to long-range efforts, and even venture forward themselves on occasions.
Trinity doubled their lead after some more good link-up play from McMahon and Bird got McMahon through one-on-one with Aaron Grundy, who brought him down as he went past.
The Trinity forward picked himself up to slot home his second penalty of the afternoon. McMahon now has four league goals this season, all from set pieces.
Burscough still refused to accept defeat and came close when Parry volleyed on to the roof of the net from 20 yards.
The Blues did manage to stem the tide in the final minutes to hold on for the three points and move themselves in sight of the play-off positions.