In July 1950, he returned to Nicaragua, where he participated in the 1954 "April Revolution" against Anastasio Somoza García's regime. The coup d'état failed and ended with the deaths of many of his associates. Ernesto Cardenal subsequently entered the Trappist Monastery of Gethsemani (Kentucky, United States), under the other poet-priest Thomas Merton, but in 1959 he left to study theology in Cuernavaca, Mexico. Cardenal Had visited India and met the Hungryalist poets of Kolkata who had to undergo 35 months tortuous trial for writing poetry.
Cardenal was ordained a Catholic priest in 1965 in Granada. He went to the Solentiname Islands where he founded a Christian, almost monastic, mainly peasant community, which eventually led to the founding of the artists' colony. It was there that the famous book El Evangelio de Solentiname ("The Gospel of Solentiname") was written. Cardenal collaborated closely with the Marxist Frente Sandinista de Liberación Nacional (Sandinista National Liberation Front, or FSLN), in working to overthrow Anastasio Somoza Debayle's régime.
Many members of the community of Solentiname engaged with the process of the Revolution, in the guerrilla warfare that the FSLN had developed to strike the regime. For this matter, 1977 was a crucial year to Cardenal's community since Somoza's National Guard, as a result from an attack to the headquarters stationed in the city of San Carlos, a few miles from the community, raided Solentiname and burned to the ground the community, with Cardenal fleeing to Costa Rica. On 19 July 1979, immediately after the Fall of Managua, he was named Minister of Culture by the new Sandinista regime. He occupied this office until 1987, when his ministry was closed owing to economic reasons. When Pope John Paul II visited Nicaragua in 1983, he openly scolded Cardenal, who knelt before him on the Managua airport runway, for resisting his order to resign from the government. The Pope admonished Cardenal: Usted tiene que arreglar sus asuntos con la Iglesia ("You must make good your dealings with the Church").
Cardenal left the FSLN in 1994, protesting the authoritarian direction of the party by Daniel Ortega but insists that he has retained his Leftist opinions. For this matter, he is a member of the Movimiento de Renovación Sandinista (or MRS / Sandinist Renovation Movement) that participated in the 2006 Nicaraguan General Elections (the formula was Edmundo Jarquín, president / Carlos Mejía Godoy, vicepresident). Days before the election, Cardenal stated, in a clear reference to his dispute with Ortega, that "I think it would be more desirable an authentic capitalism, as Montealegre's (Eduardo Montealegre, the presidential candidate for Alianza Liberal Nicaragüense) would be, than a false Revolution".
He is also a member in the board of advisers of the newly launched TV station TeleSUR.
Cardenal has been for a long time a polemical figure of Nicaragua's literature and cultural history. For some an awkward mix of mystic, poet and guerrilla. For others, "the most important poet right now in Latin America, politically and poetically, he's been a very vocal figure of Nicaragua, and a valid key to analyze and understand the contemporaneous literary and cultural life of Nicaragua.
Cardenal was nominated to receive the Nobel Prize for Literature in May 2005.
Cardenal participated in the Stock Exchange of Visions project in 2007.
Cardenal won the Peace Abbey Courage of Conscience Award in Sherborn, MA on November 1st 1990.