His first major post was that of a French ambassador to the Republic of Venice (from 1627). Later, count d'Avaux headed a diplomatic mission to Denmark and Sweden in 1634. In 1635, he contributed to the achievement of an armistice between Sweden and the Polish-Lithuanian Commonwealth in the Treaty of Sztumska Wieś, where he met many Polish magnates (such as Rafał Leszczyński). Count d'Avaux concluded treaties between France and Sweden in 1638 and 1641, and during the years 1643–1648, he took part in the lengthy negotiations which led to the Peace of Westphalia, ending the Thirty Years' War.
Comte d'Avaux was Frances's ambassador to the Republic of Venice when the Ring of Fire deposited Grantville in Germany. Initially, he saw little cause for concern, until the true nature of the Americans reached him. D'Avaux regarded France's enemies as a scourge of God, a view that extended to Grantville. Cardinal Richelieu gave orders to d'Avaux to discourage the American embassy to Venice, so as to ensure the strategic isolation of the United States of Europe.
D'Avaux had sent his loyal Huguenot Michel Ducos to spy on the Americans and their allies. When Ducos suggested staging a fake assassination attempt on Pope Urban VIII during Galileo's trial, and placing the blame on the Americans, d'Avaux agreed with the plan. Unknown to d'Avaux, Ducos used his position to further the Huguenot cause, and really intended to kill Urban. Ducos further planned to frame d'Avaux for the assassination and lay the blame on France.
After receiving news of Lawrence Mazzare's elevation to cardinal and Michel Ducos' failed attempt to assassinate the Pope - all of which damaged France's reputation and its relationship with Venice, d'Aveax drank himself into a stupor in his wine cellar. When the news reached Cardinal Richelieu, he demoted the Comte d'Aveax to a scut-post in Brittany in order to contain the damage done to France.