Through one of her colleagues at the advertising agency, Belli met Camilo Ortega, who introduced her to the Sandinistas and asked her to join the group. In 1970, Belli joined the struggle against the Somoza dictatorship, sworn into the movement by Leana Ortega, Camilo Ortega's wife. Belli's work for the movement led to her being forced into exile in Mexico in 1975. Returning in 1979 just before the Sandinista victory, she became FSLN's international press liaison in 1982 and the director of State Communications in 1984. During that time she met Charles Castaldi, an American NPR journalist, whom she married in 1987. She has been living in both Managua and Los Angeles since 1990. She has since left the FSLN and is now a major critic of the current government.
In 1970, Belli published her first poems in the literary supplement of Nicaraguan newspaper La Prensa. In 1972, she won the Premio de Poesía Mariano Fiallos Gil award from the Universidad Nacional Autonoma de Nicaragua.