Nadir Ciftci netted twice before Gavin Gunning's penalty, while Gary Mackay-Steven and Stuart Armstrong both scored after the break.
And a woeful day for Caley Thistle was further compounded by red cards for Greg Tansey and Marley Watkins.
Ciftci opened the scoring on 16 minutes as he latched on to a lofted pass from John Rankin and nodded the ball past the advancing Dean Brill to slot home from a tight angle despite the best attempts of Gary Warren.
Inverness had defended that goal badly, but a bigger calamity followed when a back pass from Josh Meekings slowed on the sanded pitch and he then got in the way of Brill's clearance allowing Ciftci the simple task of tapping the ball over the line.
And the hosts were in further trouble eight minutes later when Graeme Shinnie clipped Gunning just inside the box and the United defender blasted the resulting penalty high into the net.
A minute before the break Tansey was dismissed when he went over the ball in a challenge with Paul Paton.
Mackay-Steven secured victory for United on the counter-attack as he raced on to a pass from Armstrong and slipped the ball under Brill four minutes after the break.
Armstrong (57) added the fifth when he got in front of Warren to meet a Keith Watson cross from the right and power his header past the keeper.
And with 18 minutes remaining Watkins slid in on Paton with his studs showing and referee Willie Collum produced another red card.