During the Axis Occupation of Greece in the Second World War (1941–44), Peponis was an active member of two resistance organizations: the Panhellenic Union of Fighting Youths (PEAN) and the National Coalition of Higher Education Institutions (ESAS), being involved especially in the underground press.
As the general director of the Hellenic Broadcasting Corporation in 1964-65 he founded the Experimental Channel which began Greek public television.
During the Greek military junta of 1967–1974 he was arrested five times, imprisoned, held in solitary confinement, and ordered into exile.
[6] As the minister responsible for energy policy he conceived of, negotiated and signed an agreement on importing natural gas to Greece from the then-Soviet Union and Algeria and started its realization in 1987–88.
In the same capacity, he proposed and oversaw a strategy for disengaging foreign companies searching for oil in the northern Aegean from issues of national security and international policy.