Mary wore a stylish tasseled gown for the occasion which featured a split at the neckline.
The princess accessorised with drop earrings and a bright blue clutch bag.
Her appearance came as the Palace confirmed Prince Frederik had undergone surgery for an issue with his back.
The procedure had gone according to plan, the palace said on Monday.
Crown Prince Frederik has since been discharged from hospital and will spend time recovering.
The Prince has previously spoken about issues with his back – although it is not clear if these are related.
‘I have had a few back problems lately which have stopped me from going running as I would like to,’ he said in a video media following his 50th birthday celebrations.
It has been reported that the Prince will not undertake official engagements for two weeks.