Mussomeli's grandparents on both sides of the family immigrated to America from Sicily around the turn of the 19th century.

He graduated from Camden Catholic High School in 1970, going from there to Rutgers University-Camden for two years before taking time off to hitch-hike through Europe.

His next overseas assignment was in Embassy Manila, Philippines, as a consular officer from 1984 to 1986, to which he would later return in 2002 as Deputy Chief of Mission (DCM).

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.

After 2011 elections he offered his help in forming government coalition, for which he was criticised by then President of Slovenia Danilo Türk.

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.