Born on June 8, 1942 in the village of Krestena near Olympia, Konstantopoulos studied law in the University of Athens. During his period as a law student he became actively involved in the student movement as a member of the Center Union .
During the 1967-1974 dictatorship, his ideas became more radical. He was a member of the Democratic Defense (Greek: Δημοκρατική Αμυνα) anti-junta resistance group. He was arrested, tortured and sentenced in 8 years of imprisonment by the regime in 1970.
Ηe was charter member of the Panhellenic Socialist Movement (PASOK) in 1974. One year later, he was expelled due to his disagreement with the party's leader Andreas Papandreou. Together with professor Sakis Karagiorgas, his partner from the resistance, he founded the party of Socialist March (Greek: Σοσιαλιστική Πορεία). Konstantopoulos served as the spokesman for the new-founded party. He took part in the 1977 election as a member of the short-lived Socialist March within the Alliance of the Left and Progressive Forces (Greek: Συμμαχία των Αριστερών και Προοδευτικών Δυνάμεων).
He became a founding and leading member of Synaspismos in 1989. In the same year, he was elected member of the parliament and served as Minister of Internal Affairs in the government of Tzannis Tzannetakis, in an alliance with the conservative party of New Democracy. This unusual left-conservative alliance, plus the fact that Konstantopoulos was one of the prosecution lawyers in the trials of Andreas Papandreou and many others of his former PASOK companions, made Synaspismos and personally Konstantopoulos targets of severe criticism.
In the 1993 Hellenic Parliament election, the failure of Synaspismos to pass the 3% threshold in order to enter the parliament was a near disaster for the party. Maria Damanaki, who was then the president of Synaspismos resigned from her position, and Konstantopoulos was elected as the party leader. He soon became very popular, being among the top in popularity polls. In the 1996 election, Synaspismos re-entered the parliament with a percentage of 5,2% countrywide, a success credited to a large extent to Konstantopoulos himself.
In the 2004 Hellenic Parliament election, Synaspismos narrowly escaped from being excluded from the parliament again, acquiring a disappointing 3,2% at a national level, despite it had formed an alliance with other minor parties of the Greek left. This alliance became inactive in a few months' time, failing to participate united in the same year's European Parliament election. Konstantopoulos received criticism from both his party's members and his left allies for the two consecutive failures, and announced that he would not be candidate for president in the next Synaspismos congress.
The Synaspismos congress of December 2004 elected Alekos Alavanos as the next president of the party, the third following Damanaki and Konstantopoulos.
Konstantopoulos, throughout his political carrier, has also been active as a professional lawyer, specializing in various issues of institutional reforms, law modernization, criminal law and the defense of civil rights.