The face-lift has restored the 17th-century palace, empty since the death of his grandma Ingrid in 2000, to its former glory.
For the last five years, the couple have been living in the modest Chancellery House at Fredensborg Palace, 40km north of the Danish capital.
In their new home, walls, ceilings and floors have all had a makeover, while the electrics have also been updated.
The interior also now boasts gold trim, lining columns, doors and mirrors.
Meanwhile, an extra four bathrooms have been constructed, bringing the total to 20.
To add a modern touch, eight young local artists were chosen to decorate a room each.
In a nod to Mary's Australian roots, a roof terrace measuring 70sq metres has also been built so the royal couple can entertain friends over barbecues.
From next February the public will be allowed to see the results, though the family's private quarters will remain off-limits.