In the post-war era, he became a prominent Athens lawyer, and was closely associated with Georgios Papandreou, the leader of the Liberal Party and later of the Centre Union.
As a close associate of Andreas Papandreou, Koutsogiorgas became a founding member of the Panhellenic Socialist Movement (PASOK) during the metapolitefsi.
[2] During this time he played a major role in the March 1985 political crisis over the issue of the re-nomination of then-President of Greece Constantine Karamanlis for a second term.
[5] While popular with the broader PASOK electorate but lacking a strong party base, this appointment made him the de facto number two in the government.
During Papandreou's illness in 1988 and his absence from Greece in August–September for a heart bypass operation in the United Kingdom Koutsogiorgas functioned as the virtual Prime Minister.
[6][7] On 18 May, the PASOK Central Committee voted against him standing as a candidate during the June 1989 parliamentary elections until the investigation on the Bank of Crete case was complete.
[3] On 27 September, in a Parliament vote Koutsogiorgas along with Andreas Papandreou, Dimitris Tsovolas and Giorgos Petsos were sent to a Special Tribunal set up to investigate the Koskotas scandal.
In a move that at the time was criticized as politicized and excessive, the 68-year-old Koutsogiorgas, alone among the accused, was placed into pre-trial custody at Korydallos prison on 2 October 1990.
[9] On 11 April, while examining a witness (Stathis Papageorgiou, former Deputy Governor of the Bank of Greece), Koutsogiorgas collapsed in the courtroom and died seven days later.
His funeral gathered large crowds in the center of Athens and developed into a political protest against the continuing proceedings of the Special Tribunal.