In 1940–1941 he fought with the Greek Army in the Greco-Italian War as a second lieutenant of Infantry, and was wounded in battle.
Later, after the German invasion of Greece, Yannopoulos joined the left-wing National Liberation Front (EAM) and served as an officer in its military branch, ELAS.
After the dictatorial Regime of the Colonels took power in Greece in 1967, Yannopoulos was active in the resistance against the dictatorship, and was imprisoned.
In 1974, after the fall of the Colonels, Yannopoulos played a key role in arranging criminal prosecutions against them, along with fellow lawyers Alexandros Lykourezos, Grigoris Kasimatis, and Foivos Koutsikas.
For many years Yannopoulos had his own political talk show on Greek television, titled "I Alli Opsi" (The other view), which achieved very high ratings.