After studying Law and Economy at the University of Cluj (1930–1934), he engaged in local politics with the PNR's direct successor, the National Peasants' Party (PNȚ), and worked as a journalist; he wrote for România Nouă, edited by Zaharia Boilă, Mesajul (Zalău), Unirea (Blaj).

[3] Accused of propaganda against the National Rebirth Front (Frontul Renașterii Naționale), Coposu was sent into forced domicile in Bobota.

After the Second Vienna Award of August 1940, when Romania was forced to cede Northern Transylvania to Hungary, Coposu moved to Bucharest.

[5] In 1945, after the royal coup against the Antonescu regime, Coposu became deputy secretary of the PNȚ and, after the reintegration of Northern Transylvania, the party's delegate to the leadership of provisional administrative bodies.

His mentor, Iuliu Maniu, the leader NPP, the most important political organization in Romania, received a life sentence in a show trial.

Coposu later attested that his imprisonment, imposed by Soviet officials overseeing the Securitate, was among those causing a stir in the higher echelons of the Communist Party.

Belu Zilber, a Communist who was purged together with Lucrețiu Pătrășcanu, later told him that prominent party politician Ana Pauker had unsuccessfully opposed the move in front of Gheorghe Gheorghiu-Dej.

[12] After his release, Coposu started work as an unskilled worker on various construction sites (given his status as a former prisoner, he was denied employment in any other field), and was subject to Securitate surveillance and regular interrogation.

[13] After the collapse of communism, Tudor Călin Zarojanu published large excerpts from the huge Securitate file on Corneliu Coposu, kept for decades by the secret communist political police [14] His wife Arlette was also prosecuted in 1950 during a rigged espionage trial, and died in 1966, soon after her release, from an illness contracted in prison.

The Prime Minister Petre Roman addressed the angry mob who wanted to lynch Coposu and the other leaders of the democratic opposition, pretending to mediate the conflict.