In 1979, Fuster was named Deputy Assistant Attorney General of the United States. He held that position until 1981. In 1984, he was elected Resident Commissioner of Puerto Rico to the United States Congress. During his tenure (1985–1992), he served a term as Chair of Congressional Hispanic Caucus. In both terms, he served on the Committee on Banking, Finance, and Urban Affairs, and on the Committee on Interior and Insular Affairs. In Congress, he was a strong advocate for educational and youth programs. He supported numerous laws and bills to give state and local governments assistance for youth service projects and programs aimed at preventing substance use. He also supported the establishment of a Children, Youth, and Families Administration, as well as the establishment of a federal child care program. He sponsored legislation to increase social security funds for families with blind, aged, and disabled dependents.
In 1992, Governor Rafael Hernández Colón appointed him to the Supreme Court of Puerto Rico. Fuster resigned from his Resident Commissioner position on March 4, 1992 to take an appointment as associate justice of the Supreme Court of Puerto Rico. He took the oath of office on March 4, 1992 after confirmation by the Senate, serving on the Court until his death.
Fuster died during the early hours of December 3, 2007 from cardiac arrest. He was buried at Buxeda Memorial Park Cemetery in Río Piedras, Puerto Rico. His death left the Supreme Court in an unprecedented position with two seats vacant.
