Playing against last-placed Rotherham, Huddersfield were expected to cruise to another three points at the New York Stadium, but found themselves trailing 2-1 with less than 20 minutes left.
That’s when Mooy came into his own, setting up Huddersfield’s equaliser as his pinpoint corner was headed in by Elias Kachunga.
Then in stoppage-time, skipper Tommy Smith scored with an excellent half volley to hand the Terriers a last-gasp 3-2 win.
It’s a huge result for Mooy and Huddersfield who consolidated third spot on the Championship ladder, just five points off the top and four points from an automatic promotion slot.
Elsewhere in the Championship, defender Bailey Wright skippered Bristol City again as they lost 2-1 at promotion-chasers Leeds United.
Young defender Callum Elder played 55 minutes and Ryan Williams came off the bench late as Barnsley won 3-1 at Aston Villa.
Caltex Socceroo skipper Mile Jedinak was again sidelined for Villa with the groin injury which has kept him out of action for a month.
Villa boss Steve Bruce is hopeful Jedinak will be fit for the weekend’s clash with league-leaders Newcastle United.
In League One, Aiden O’Neill was in midfield for Oldham Athletic who held on for a vital 1-0 win over Charlton to pull away from the relegation zone.
Andy Rose played a full match for Coventry who conceded in the third minute of stoppage-time to draw 1-1 at AFC Wimbledon.
Left-back James Meredith played a full match for Bradford City as they lost 2-1 at Fleetwood Town, while defender Cameron Burgess helped Bury keep a clean sheet in a goalless draw with MK Dons.