Ambassador Gardner serves as senior advisor to Sidley, the international law firm, and Brunswick Global, the public affairs and communications firm. He is a board member of Scottish Power and Brookfield Business Partners.

In addition, he serves as visiting distinguished fellow of The German Marshall Fund of the United States (GMF) and is a member of the Strategic Council of the European Policy Centre. He is an advisor to the Bill and Melinda Gates Foundation and is a member of the Council on Foreign Relations.

Ambassador Gardner served as U.S. Ambassador to the European Union from March 3, 2014 to January 20, 2017. He has dedicated more than 25 years of his career to U.S.–European affairs, as a government official, lawyer, and investor. He served as director for European Affairs on the National Security Council in 1994–95. Prior to serving in his diplomatic post, Ambassador Gardner was managing director for six years at Palamon Capital Partners, a pan-European private equity firm based in London, and worked as an executive director at GE Capital and Bank of America, as well as with international law firms in Brussels, Paris, London, and New York. He is a member of the New York Bar.

He has written a book and numerous articles on EU affairs and U.S.–EU relations, and is a frequent commentator on CNN, BBC, CNBC, and Bloomberg, as well as in the Financial Times and the Wall Street Journal.