The Socceroos veteran kept City in control of the race for third place on the ladder, and a home final, with a trademark first-half header against the Jets.
Nick Fitzgerald, who replaced him as a second-half substitute, doubled their lead with a delightful chip before Bruno Fornaroli's late double.
But Cahill stole the show on the way from AAMI Park to two 2018 FIFA World Cup qualifiers in the next fortnight.
The 37-year-old hung in the air on the back of young defender John Koutroumbis, powerfully heading past goalkeeper Jack Duncan.
With Lleyton Hewitt applauding in the crowd, Cahill departed from his normal corner flag-punching celebration to serve an imaginary a tennis ball in front of the former world No.1.
The goal would be music to the ears of national team boss Ange Postecoglou, as he readies the Socceroos to take on Iraq and United Arab Emirates in vital matches on the road to Russia.
Perhaps with an eye to his national team duty, Cahill was substituted after 55 minutes.
But the forward didn't have eyes for the substitutes board, seeing his number called but refusing to budge - eventually beginning a slow trudge to the bench.
With Cahill departed, City found the goals their dominance deserved.
Fitzgerald latched onto a Anthony Caceres' inspired ball to volley over Duncan on 65 minutes.
Then fellow substitute Bruce Kamau danced through the box to draw a foul from Daniel Mullen and give Fornaroli the chance from the spot.
He scored with ease, before raising a t-shirt bearing team-mate Fernando Brandan's name to recognise the ACL injury suffered last week by the Argentinian.
Fornaroli netted his 17th goal of the season when a loose ball in the box fell to him under no pressure.
He should have ended the night on four goals, only for Duncan to deny the Uruguayan after crosses from Fitzgerald and Kamau.