Played at an Al-Sadd Stadium packed with noisy expatriate Indians, a convincing win was needed by the Socceroos to demonstrate their tournament credentials against a team ranked 142 in the world.
They comfortably delivered with the dangerous Cahill, playing up front as the lone striker, a constant threat.
He got the opener in the 11th minute before former Liverpool man Harry Kewell made it 2-0 14 minutes later.
Brett Holman added a third on the stroke of half-time before Cahill completed the demolition in the second period.
Their start was in stark contrast to four years ago at their maiden Asian Cup, where they stumbled to a draw with Oman before losing to eventual winners Iraq.
The Australians squeezed through to the knock-out stages back then by thrashing Thailand 4-0, but went out on penalties to Japan in the quarters.
In breezy conditions, both sides began slowly with Australia getting the first sniff of goal on nine minutes when Holman looped the ball in to Blackburn's Brett Emerton, who fired straight at the goalkeeper.
But with a side packed by experienced Europe-based players, it was only a matter of time before the Socceroos got the breakthrough.
Emerton looked suspiciously offside when he was put through on the right but the flag stayed down and he sent a low cross into the six-yard box, where Cahill was lurking to bury the ball in the back of the net.
Rank outsiders India, in the tournament for the first time in 27 years, were out of their depth but got a look at goal when captain Climax Lawrence's shot from 25 yards drifted well wide of the post soon after.
Australia should have been 2-0 up on 20 minutes when Cahill headed home but he was adjudged offside by UAE referee Ali Al Badwawi. Replays suggested the goal should have stood.
They made amends just minutes later when Luke Wilkshire found Kewell in space just outside the penalty area and the Galatasaray midfielder clinically drilled the ball into the bottom left-hand corner.
With Fulham's Mark Schwarzer having nothing to do in the Australian goal, a third goal almost came on the half-hour with Cahill in the thick of the action again.
India were struggling to contain him and he went close once again when he chested down a long ball and half-volleyed it just over the bar.
The third goal was inevitable and it came just before half-time when Holman got his head on the end of an Emerton cross after some good build-up work.
Australia began the second-half in total control and Cahill got his second in the 65th minute when he outjumped four defenders to meet a Wilkshire cross and angle his header past the goalkeeper.
India, with Englishman Bob Houghton in charge, now have their work cut out with Group C clashes against mighty South Korea and a very capable Bahrain still awaiting them.
Australia face the Koreans on January 14 in a match that could determine who finishes top of the group.