He was described as a tall, lean, handsome, cool eyed, suave and rich, but had a weak body.
At two, he lost his mom, and was left by his father. He was brought up by his uncle. His brought up made him cold and timid. He chose an opera singer to be his mistress. He did not marry.
He was Austria Ambassador. He was also responsible to teach young Marie Antoinette when she first married to France Dauphin in 1770. He kept on writing letters described incidents and overheard conversations about Louis and Antoinette’s private life to Empress Marie Theresa in Austria, so that Marie Theresa could guide her. Marie Theresa simply told Marie Antoinette that she "read in the newspaper." So, Marie Antoinette did not know Mercy was the spy.
Mercy understood that he would be indispensable to the Empress only by showing how influential he was in the court. Therefore his letters often described Louis was a weak person and controlled by Marie Antoinette, so that he would appear strong in Vienna that could control the future King of France.
He remained as a constant correspondence with Marie Antoinette after the royal family moved to Paris in 1789.