Former Dutch international midfielder De Jong, who played in the 2010 World Cup final defeat by Spain and won the 2012 Premier League title with Manchester City, had an afternoon to forget in Hanover.
The 33-year-old defensive midfielder, who made 66 Bundesliga appearances for Hamburg between 2006 and 2009, has signed for Bundesliga strugglers Mainz until the end of the season.
It all started so well for Mainz as a goal by Japan striker Yoshinori Muto and Alexander Hack’s header put them 2-0 ahead with half an hour gone.
But after Hanover striker Niclas Fuellkrug made it 2-1 with a header towards the end of the first half, De Jong brought down midfielder Felix Klaus in the area and the referee pointed to the spot.
To De Jong’s dismay, Fuellkrug netted the resulting penalty on 38 minutes before banging home the winning goal 15 minutes from time to complete his hat-trick.
The defeat leaves Mainz 15th in the table, just a point above the bottom three, while Hanover climb to 10th.
There was a happier return for Germany striker Mario Gomez, back at Stuttgart after an eight-year gap, as he helped seal a 1-0 win at home to Hertha Berlin.
With time running out, former Bayern forward Gomez was brought down in the area by Hertha’s Niklas Stark.
While the German international was busy appealing for a penalty, the ball looped off Stark’s boot and into the net for the winning own goal on 78 minutes much to Gomez’s delight.
The result leaves Stuttgart 12th, one place below Hertha, who hit the post through Austria international Valentino Lazaro in the first half.
Elsewhere, Augsburg climbed to seventh with a 1-0 win over strugglers Hamburg thanks to a winning header by South Korea international Koo Ja-Cheol just before the half-time whistle.
Eintracht Frankfurt blew the chance to jump from ninth to third after French striker Sebastien Haller gave them a lead at home to Freiburg, who equalised through a header by defender Robin Koch in the second half in a 1-1 draw.
Werder Bremen and Hoffenheim also shared a point in their 1-1 draw as Czech international Theodor Gebre Selassie’s second-half goal for hosts Bremen cancelled out Benjamin Huebner’s first-half header.
Bundesliga leaders Bayern Munich went 14 points clear on Friday, when the German league resumed after the winter break, with a 3-1 win at Bayer Leverkusen.
Second-placed Schalke have the chance to cut the gap when they play RB Leipzig away later on Saturday.