When his elder sister, Jeanne-Antoinette Poisson became, in 1745, the official mistress of Louis XV and was given the title "marquise de Pompadour", she had him follow her to the court, where the young man attracted the favours of the king.

With Coypel's help, Poisson de Vandières chose paintings from the royal collection for exhibition at the Palais du Luxembourg, thus creating the first museum in France.

Between December 1749 and September 1751, he spent twenty-five months in Italy, staying first at the Académie de France à Rome, and then travelling (the so-called "Grand Tour") across the country with the engraver Charles Nicolas Cochin, the architect Jacques-Germain Soufflot and the abbé Leblanc.

Irritable, boastful, easily angered, insecure about his humble origins, Marigny was nevertheless an intelligent and energetic administrator concerned with the importance of his work.

He gave numerous commissions to François Boucher, Charles-Amédée-Philippe van Loo, Jean-Baptiste Pierre and named Charles-Joseph Natoire as director of the Académie de France à Rome.