City made a near perfect start, scoring in the third minute through a well worked set-piece move that saw Cahill tap home a low and precise pass from Luke Brattan.
The Mariners looked sluggish in the opening minutes, possibly due to the fact that it had taken them one hour and a half in peak-hour Australian Open-affected Melbourne traffic, to get to AAMI Park from their hotel, which was just under five kilometers away.
Despite the early domination, the Mariners did equalise through Roy O'Donovan in the ninth minute.
The Irish striker competently finished off his eighth goal of the season a Connor Pain cut-back after earlier good work in midfield by Nick Montgomery.
But O'Donovan went from hero to zero for the Mariners in the 23rd minute.
As the Irishman attempted to contest for a long-ball, he swung his arm into the face of Melbourne City defender Michael Jakobsen. After conferring with his assistant, referee Daniel Elder showed O'Donovan a straight red card, meaning the Mariners would have to play the remaining 70 minutes with a maximum of 10 men.
Sensing an opportunity, City poured forward and created numerous chances but it was Cahill who provided the breakthrough with his second of the match and seventh goal of the season.
After a goal-mouth scramble the Mariners were able to clear the ball only as far as Brattan, who for the second time of the night picked out Cahill, this time with a classy right-foot cross to the back post, who to no-one's surprise headed home.
In the second half City continued to control the match and the margin could have been more had Mariners 'keeper Paul Izzo not made one of his eight saves.
Nicolas Colazo hit the post twice and substitute Fernando Brandan missed a free header from two metres out. The shots' count at the end of the night read 26-2.
The result was the perfect tonic for Michael Valkanis' men, desperately needing a response to last week's loss to last-placed Adelaide United.
With the win City stay in third, six points behind cross-town rivals Melbourne Victory who have a game in hand, while the Mariners remain on the bottom of the table, eight points off Perth Glory in sixth.