He worked at a fishing company named Congel in the island of São Vicente.
Between 1973 and 1974, he was secretary general of the Barlavento Commercial, Industrial and Agricultural Association.
After independence in 1975, he became member of the PAIGC and PAICV party and was deputy of the National Assembly in 1975, 1980 and 1985.
Fonseca has taken a position as the ambassador to the Netherlands and European Community from 1987 to 1991, then Russia (1991-1994), in Ukraine[1] (1993-1994) and the Commonwealth of Independent States (CIS) (1991-1994), to Austria (1999-2001) and the United Nations (2001-2004).
From July 2004 to 2008, da Fonseca was the executive secretary of the Community of Portuguese Speaking Countries.