She worked extensively as a voice actress for the English-language dubbing of several hundred foreign (mostly Italian) films from the early 1960s and onwards.
She also had bit part roles in some big productions that did shooting in Italy, such as Barabbas (1961) and The Pink Panther (1963).
Subsequently, De Fonseca dubbed a series of further peplum film and she was also given the chance to dub the voice of American actress Jayne Mansfield in two of her European films which were post-synchronized without Mansfield's involvement: Primitive Love (1964) and Dog Eat Dog (1964).
She specialized in voicing femme fatale characters such as the evil queen (played by Jany Clair) in Hercules vs. the Moon Men (1964), a bitchy tourist (played by Silvia Solar) in Eyeball (1975) and the deranged inmate Albina in Women's Prison Massacre (1983).
On screen she played Christopher Reeve's secretary in Monsignor (1982), had a supporting role in the Pia Zadora film The Lonely Lady (1983), played a comedic role as an American tourist in Detective School Dropouts (1986) and finally appeared in Bernardo Bertolucci's The Sheltering Sky (1990).