Appointed president of the Institute of Retirements and Pensions for Stevedores and Cargo Transport (IAPETEC) in 1963, he was appointed deputy on 11 April 1964 after the removal from office of Adalil Barreto by the Institutional Act Number One issued by the military regime.

[1] With the arrival of enforced bipartisanship via the Institutional Act Number Two, he joined National Renewal Alliance (ARENA) and was successively re-elected[2] in 1966, 1970, 1974, 1978 and 1982.

In 1986 he ran for a new term as federal deputy but was left as a substitute, however his family was represented in the Brazilian Constituent Assembly that drafted the Constitution of 1988 by his niece, Moema São Thiago.

Marcílio assumed his last term as federal deputy on 23 May 1990, days after the resignation of Luís Marques, who was appointed general director of the National Department of Works Against Droughts (DNOCS) by president Fernando Collor.

[1] He was the only native of Piauí to preside over the Chamber of Deputies and also the only one born in the state to compose a presidential ticket; Franklin Dória, born on Ilha dos Frades, Bahia, was the only representative of Piauí to preside the Chamber of Deputies.