MK Dons boss Karl Robinson was beaming after his side ruined Nigel Adkins' first game in charge of Southampton.
Substitute Angelo Balanta's neat 72nd-minute finish and Peter Leven's close-range strike condemed Saints to a 2-0 defeat and maintained the Dons' 100% home record.
Robinson said: "It means a lot to us, that was a very good team we beat there.
"They've got possibly some of the strongest players in this division, if not the Championship, with a fantastic manager in charge.
"It was a big win - it was a very workmanlike performance and it's one we've got to enjoy."