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.

1988, Belli’s book La Mujer Habitada (The Inhabited Woman), a semi-autobiographical novel that raised gender issues for the first time in the Nicaraguan revolutionary narratives, brought her increased attention; this book has been published in several languages and was on the reading list at four universities in the United States. The novel follows two parallel stories: the indigenous resistance to the Spanish and modern insurgency in Central America with various points in common: women’s emancipation, passion, and a commitment to liberation. In 2000, she published her autobiography, emphasizing her involvement in the revolutionary movement, El país bajo mi piel, published under the name The Country Under My Skin in the United States; it was a finalist for the Los Angeles Times Book Prize in 2003. Belli continues publishing and maintains that poetry is her most important work. Belli was the recipient of the Premio Casa de las Américas in 1978. In 2008 Belli received the Premio Biblioteca Breve for her book El infinito en la palma de la mano (Infinity in the Palm of The Hand), an allegory about Adam and Eve in paradise.

Her 2010 book was submitted with the title “Cronicas de la Izquierda Erótica”, but had to be changed to “El País de las Mujeres”, since the previous title was too similar to that of a 1973 book by Ana María Rodas: Poemas de la Izquierda Erótica. The book tells the story of a world governed by women. In the novel she portrays a group of women that take power by means of a Political Party named “Partido de la Izquierda Erotica”. This is the same name as a movement formed by women during the 80s, to which Belli belonged, which had been named as a tribute to Rodas´ work.

In 2018, Belli took a stand against the government of Daniel Ortega, which emerged from the 2016 elections, and became an active member of the Sandinista renewal movement.