Widely dismissed as contenders for relegation before they had even kicked a ball, they produced a typically-energetic display to outclass their experienced hosts, earn three vital points and climb to first place.
After an unremarkable opening period, it was the visitors who first began to threaten.
Huddersfield had an early chance to take the lead but Tom Ince side-footed Elias Kachunga's cross straight at Wayne Hennessey in the third minute of the match, however, Huddersfield did take the lead in the 23rd minute.
With time and space on the left wing, Mooy sent an accurate cross into Palace's area from where Mounie was left unchallenged to routinely head his first goal for the club.
He leaves the professional game as a player with 460 games to his name, with a majority of his career being in the Premier League and Cardiff City, with clubs like Charlton Athletic, Crystal Palace, Cardiff City and Fulham to name.
De Boer is an interesting appointment for the Eagles, with the shock departure of former England boss Sam Allardyce allowing them to bring in a more exotic appointment.
"When the chairman sat next to me and spoke about my signing he said we want to be solid club like Southampton, step by step and now they are fighting for a European place every year", De Boer said.
It didn't take long for Huddersfield to double their advantage.
The substitute combined well with debutant Ruben Loftus-Cheek to create space to send a low cross towards Christian Benteke that remained just beyond the striker's reach when he had only needed to tap in. He returned it back across the goal where Ward could only put into his own net to break the deadlock.
Scott Dann should have halved the deficit moments before Mounie settled the contest 12 minutes from time, the 30-year-old blazing over his six-yard effort before Huddersfield broke down the other end.
The opening goal came from a poorly defended corner when Christopher Schindler rose highest to glance on Kasey Palmer's inswinging delivery before Danish defender Zanka's prod at the back post was bundled in by the wrong-footed Ward.