Prior to doing so, he received a PhD in Modern European History from Vanderbilt University, was a Fulbright-Hays Research Fellow in Italy, taught modern European history at Stanford University, and served as an officer in the California Army National Guard.

He saw diplomatic overseas positions at the US Embassies in The Bahamas, Chad, and Haiti.

He also held positions in the Department of State's Operations Center, Policy Planning Council, Office of Western European Affairs, and Office of Canadian Affairs in Washington D.C. Nigro won the Department of State's Superior Honor Award for his service in Haiti.

On September 4, 2007, Nigro was nominated by President George W. Bush to be the United States Ambassador to Chad.

He is the author of The New Diplomacy in Italy: American Propaganda and U.S.-Italian Relations, 1917–1919 and of scholarly articles on historical and diplomatic themes.