Perhaps mindful of the time they first met nearly 20 years ago, they looked out onto the world's most romantic city through the face of a giant clock.
The couple, who met while studying for a degree in history of art at St Andrew’s University in Scotland, toured the gallery on the second day of their two-day visit to Paris.
It remained open to the public throughout, prompting gasps from tourists who crowded round to take pictures and videos of the royals on their phones.
Home to some of the greatest works of French and European art produced in the 19th and 20th centuries, they had specifically asked to see one some of Claude Monet’s most famous paintings including one of his water lilies series, painted in 1904.
The French impressionist painted around 250 oil paintings of the flower garden at his home in Giverny, which were the main focus of his artistic output during the last 30 years of his life.
The couple were also shown other Monet masterpieces, including his 1873 work Coquelicots or wild poppies and his "parasol" paintings from 1886 titled, Essai de figure en plain-air femme a l'umbrelle.
They stopped to take a close look at London, Houses of Parliament, which was inspired by Monet's 1871 visit to London when he was struck by the “effects of fog on the Thames”.
William, 34, asked director Laurence des Cars: "This is one of his most famous paintings isn't it?"
The 1904 masterpiece will go on loan to the Tate Britain later this year as part of an exhibition called: “The Impressionists in London”.
The couple were also shown Monet’s La Rue Montorgueil a Paris, a 1878 painting of a street which the Queen visited during her 2004 tour of France.
Art enthusiast Kate, 35, who graduated with a 2:1 in history of art in 2005, asked lots of questions, particularly about Edouard Manet's Olympia, a nude painted in 1863.
Seemingly unfazed by the topless scene, William pointed to a black cat in the picture and was told it was a tongue in cheek addition.
Another painting that caught their eye was Gustave Caillebotte's Raboteurs de Parquet (1875) featuring half-naked men sanding a floor by hand.
A keen photographer, Kate has been patron of the National Portrait Gallery in London since 2012 and is also patron of a charity called The Art Room, which encourages disadvantaged children to express themselves through artistic endeavours.
Kate once said: “I am a firm believer in the power of art to make a difference”.
William, whose father and grandfather are both keen artists, also studied history of art before switching to geography.
It is thought that Kate’s support in convincing him to switch subjects rather than quit university altogether is what first drew them together.
Situated on the left bank of the Seine, the Musee D’Orsay is houses in the former Gare D’Orsay, a Beaux-Arts railway station built between 1898 and 1900.
It holds mainly French art dating from 1848 to 1914 including works by Monet, Manet, Degas, Renoir, Cezanne, Gaugin and Van Gogh.
Having been used as a railway station for more than 80 years, the decision was taken to close it down because its platforms were deemed too short for modern trains.
In 1970 permission was granted to demolish the building but after an eight year row it was finally placed on the list of historic monuments and former French president Georges Pompidou gave the go ahead for it to be turned into an art gallery.
It opened in 1986, with its original clocks remaining as a reminder of its transport heritage.