According to his remembrance, he turned to poetry at 11 or 12, and from the early 1960s he published in such renowned periodicals as Kortárs and Élet és Irodalom.

During the following years he nursed at a mental clinic as a preliminary exercise for planned psychiatry studies, resigning from which he showed interest in economics and law, but later he decided to be a philosopher.

Between 1981 and 1985 he co-edited Beszélő, the illegal paper of the Democratic Opposition, and became involved in their anti-regime activities; he was a member of SZETA (Fund for the Support of the Poor, an illegal NGO that drew governmental attention by advocating the mere existence of poverty) from which a liberal party, SZDSZ was formed in 1988.

By receiving Kossuth prize in 1996 along with Péter Esterházy, he once again became subject to political criticism for alleged disrespect to Christianity.

After his death, Petri's oeuvre was re-issued in a four volume collection by Magvető Publishing House under revision by poet Szabolcs Várady, one of his closest friends.