In the process, the Demons condemned Ross Lyon's Saints and their big-name recruits Tom De Koning, Jack Silvagni, Sam Flanders and Liam Ryan to a 0-2 start to the AFL season.
All eyes were on how Melbourne, who parted ways with Christian Petracca (Gold Coast), Clayton Oliver (GWS) and Steven May (retired) over the off-season, would structure up against the Saints.
Van Rooyen, so often the face of Melbourne's dysfunctional attack under Simon Goodwin, was the chief beneficiary of a slicker, fast-moving Demons midfield in Sunday's 18.12 (120) to 15.17 (107) win at the MCG.
The key forward had five goals and four marks by half-time while former Collingwood forward Brody Mihocek added three majors.
Debutant Latrelle Pickett provided the highlight of the day when he kicked a magnificent first goal - weaving in and out of the St Kilda defenders, taking three bounces, then bouncing through a terrific snap.
The teenager was swarmed by Melbourne players, including cousin Kysaiah Pickett, who had a decent game after spending part of the week in Darwin for personal reasons.
Max Gawn (21 disposals, 34 hitouts, one goal) won an an entertaining duel with De Koning (14 disposals, 20 hitouts) while 200-gamer Christian Salem (23 disposals) and Caleb Windsor were busy.
Former St Kilda skipper Jack Steele, unwanted last year by the Saints, had 20 disposals, five clearances and a goal.
Goodwin's papers were stamped after Nasiah Wanganeen Milera's four-goal matchwinning masterclass late last year.
King sent Koltyn Tholstrup to the St Kilda maestro, who was shut down by Magpie Harry Perryman in opening round.
Wanganeen-Milera kicked the game's opening goal and finished with three majors and 18 disposals, but couldn't repeat his heroics.
Max Hall kicked four goals, had 25 disposals and was St Kilda's best, while Marcus Windhager (32 disposals), Callum Wilkie (27 disposals) and Jack Sinclair were typically reliable.
Saint Mattaes Phillipou suffered a corked left quad and was unable to continue while Liam Ryan took a hanger in the third term but came off for treatment and later had his right shoulder heavily strapped.
Kysaiah Pickett put Melbourne in front by five points - marking the game's 12th lead change - when he snapped a set shot from the pocket and St Kilda didn't have a player on the goal line to rush it over the line just before three-quarter time.
Gawn extended the lead to 10 early in the final term then teed up van Rooyen for his sixth.
St Kilda fluffed multiple late chances before Bayley Fritsch sealed victory and Melbourne ran down the clock.
The Demons travel to face Fremantle on Saturday and St Kilda will also hit the road to take on GWS.
22°C