The hosts actually started on the front foot, but saw numerous attacks break down due to a poor final ball.
Indeed, their best opening came when a clearance from a visiting defender clattered into Rasmus Nicolaisen and ricocheted wide.
That wastefulness left the door open for the Phocaeans to seize the momentum, which they duly did when Greenwood unleashed a vicious shot from a near impossible angle on the left of the area, but somehow found the far corner.
Moments later, the striker doubled his tally for the night by making it 2-0, as he was picked out by Luis Henrique inside the box and found the back of the net with a strike that Guillaume Restes may feel he should have saved.
There was worse to come before the break for Le Tefece, who were reduced to ten men after Frank Magri’s high and late tackle on Geoffrey Kondogbia resulted in a straight red card.
Toulouse were never likely to come back from there, and the game was well and truly over shortly after half time when Roberto De Zerbi’s side bagged a third when they rounded off a clinical counter attack as Amine Harit stabbed through to Henrique, whose strike clipped Charlie Cresswell and curled into the far corner.
Greenwood then came close to completing his hat trick, although somehow lifted miles over the bar after being picked out at the back post.
It was plain sailing from thereon in for Marseille, who remain unbeaten under De Zerbi and are now unbeaten in 21 Ligue 1 head-to-head's, a run dating back to March 2012.
Toulouse did at least pull one back when Shavy Babicka beat the goalkeeper to a long ball and prodded into the empty net.
However, they have now gone winless across their opening three league fixtures for the first time in 11 years.
Flashscore Man of the Match: Mason Greenwood (Marseille)
