Brentford were the only team to beat City home and away last season, and the 'three-peat' looked on when Neal Maupay fired them into the lead.
But Foden hauled City level before the interval, headed them in front early in the second half and then completed a stylish treble to leave the visitors with their title destiny firmly in their own hands.
They may have lagged behind the pace-setters for much of the season, but with Erling Haaland and Kevin De Bruyne back from injury, and Foden in this sort of form, an unprecedented fourth consecutive Premier League crown looks well within their capability.
Haaland, making his first start in almost two months after a foot injury, was understandably rusty but he did have an early shot, from a De Bruyne cross, blocked by Ethan Pinnock.
Bees keeper Mark Flekken was then kept busy with a Bernardo Silva curler, another from Julian Alvarez and also made a fine save from Kyle Walker's piledriver.
But totally against the run of play the hosts took the lead in the 21st minute, with a goal straight out of the Brentford playbook.
Flekken actually claimed the assist, punting his goal kick towards Toney, who backed into Nathan Ake, easing the defender out of the flight path while letting the ball bounce past him to strike partner Maupay.
The suddenly prolific Frenchman knew what was coming and raced clean through, coolly finishing past Ederson for his fifth goal since the turn of the year.
Back came City but Flekken was having one of the best matches of his Bees career, denying Haaland before another spectacular stop to keep out Josko Gvardiol's drive.
The Dutchman had already made nine saves - more than in any other match since joining the Bees last summer - but he was finally beaten in first-half stoppage time when Pinnock made a mess of heading De Bruyne's cross clear.
Foden had time to control the ball on his chest before side-footing past the unfortunate Flekken.
And the goalkeeper saw all his good work undone eight minutes after the interval when he could only get a weak hand to Foden's glancing header from De Bruyne's pinpoint cross.
Brentford refused to go away quietly, though, and Christian Norgaard's shot was blocked by Ruben Dias before Toney volleyed a good chance over.
But in the 70th minute the move of the match saw Foden dummy Rodri's pass, collect the subsequent ball from Haaland and dispatch it past Flekken as City made what feels like a significant move in the title race.