Mănescu joined the PCR in 1938, while he was a student at the Academy of Economic Studies in Bucharest.

[1] In 1944, after the 23 August coup d'état, he worked together with Nicolae Ceaușescu, his future brother-in-law, in the Union of Communist Youth.

He was promoted to full membership of the Executive Committee of the PCR in December 1968 and, after holding various positions in the party and government, he became Prime Minister in March 1974, a position he held until 1979, when he retired, reportedly due to ill health.

He left the Central Committee's building by helicopter together with them on 22 December, though he had to disembark at Snagov due to too much weight in the craft.

He served two years in prison, and was set free on 12 November 1992 due to poor health.