After law school, Mussomeli spent some years working in the legal field, working as a Deputy Attorney General of New Jersey, before entering the US Foreign Service in September 1980. His first overseas posting was in Cairo, Egypt, as a general service officer. After that, he returned to Washington, D.C., where he worked at the Department of State as staff assistant to the Undersecretary for Security Assistance. His next overseas assignment was in Manila, Philippines, as a consular officer from 1984–1986, to which he would later return in 2002 as Deputy Chief of Mission. Before that time, however, his tours included: North Korea Desk Officer, Senior Watch Officer, Economic Counselor in Colombo, Sri Lanka,, Inspector for the Office of Inspector General, Political Counselor in Rabat, Morocco, Deputy Chief of Mission in Manama, Bahrain, and as a member of the Senior Seminar. During his second tour in the Philippines as DCM, he served as Chargé d'affaires for a year while the Ambassador, Frank Ricciardone, was away. On June 24, 2005, the United States Senate confirmed him as U.S. Ambassador to the Kingdom of Cambodia. He then went on to serving as the Director of Human Resources/ELCDA at the U.S. Department of State in Washington, D.C., for one year. In May 2008, he served for one year in Kabul, Afghanistan as the Assistant Chief of Mission. On September 29, 2010, the United States Senate confirmed him as the U.S. Ambassador to the Republic of Slovenia. He left Slovenia in 2015. Mussomeli has received several awards including the 2010 Arnie Raphael Award, the 2008 Presidential Distinguished Service Award, two Superior Honor Awards, one Group Superior Honor Award, and two Meritorious Honor Awards.
Controversy
During his service as Ambassador to Slovenia, he was involved in some controversies. After 2011 elections he offered his help in forming government coalition, for which he was criticised by then President of SloveniaDanilo Türk. He also stated that Slovenia gained its independence too easily and that there were not enough victims in the Slovenian War for Independence. Later he criticised Türk's candidature for the UN Secretary-General and said he divides Slovenian people for which he was called to the Ministry of Foreign Affairs for consultations.
Family
Mussomeli's wife, Sharon Flack Mussomeli, is a retired foreign service officer. They have three children: Isaac, Alessia, and Thomas.
Hobbies
In his spare time, Mussomeli enjoys writing poetry and short plays, going on walks, reading about history, and playing tennis and bocce ball.