A lawyer and law professor at the University of Lisbon and the University of Cape Verde, Fonseca served as Minister of Foreign Affairs for nearly two decades from 1991 to 2001 and 2002 to 2003 under Presidents António Mascarenhas Monteiro and Pedro Pires. He also serves as the first deputy to the Secretary General of the African Party for the Independence of Cape Verde (PAICV), the ruling party of Cape Verde.
Fonseca ran for president in the 2011 presidential election as the PAICV candidate. Elected with 54% of the votes in the second round, he assumed the presidency on 9 September 2011. He was re-elected in the 2016 presidential election, defeating independent candidate Albertino Graça in the first round with 74% of the votes. He was succeeded in 2021 by José Maria Neves.