Lambrakis was born in the village of Kerasitsa in the district of Tegea (Arcadia prefecture, the Peloponnesus). After finishing high school in his home town, he moved to Athens to enter the School of Medicine at the University of Athens.
Lambrakis was a great athlete throughout his life. He held the Greek record for long jump for twenty-four years (1935–1959). He also competed in the triple jump, the 100m and the 200m. He earned 12 gold medals in the Balkan Athletic Games, which took place annually, featuring competitors from Greece, Yugoslavia, Bulgaria, Romania and Turkey. In the Olympiad of 1936, he ended up in 11th place in the long jump.
During the German occupation of Greece (1941–1944), Lambrakis participated actively in the national resistance. In 1943 he set up the Union of Greek Athletes ("Ένωση των Ελλήνων Αθλητών") and organised regular competitions. He used the revenue from these games to fund public food-banks for the starving population.
After World War II, Lambrakis completed his medical studies and worked as a lecturer in the Department of gynaecology. He continued to help the poor by running a small private clinic for patients who were unable to afford medical care.
While not a Communist, Lambrakis's political and ideological orientation did lean towards the left. He was actively involved in the Pacifist Movement of his time, which voiced strong opposition to the Vietnam War. Lambrakis acted politically from within the United Democratic Left (Eniaia Democratiki Aristera — EDA), the only legal left-wing political party in the country after the Greek Civil War 1946–1949) and until the fall of the military dictatorship. He was elected to the Greek Parliament in the 1961 elections as a Piraeus MP.
The same year (1961), under his initiative, the Commission for International Detente and Peace (Eπιτροπή για την Διεθνή Ύφεση και Ειρήνη — Epitropi gia ti Diethni Ifesi kai Irini — EDYE) was established in Greece. Under his capacity as Vice President of EDYE, Lambrakis participated in international pacifist meetings and demonstrations, despite frequent threats against his life. On 21 April 1963, the pacifist movement in Greece organised the First Pacifist Rally from Marathon to Athens. The police intervened, banned the rally and arrested many demonstrators (Mikis Theodorakis among them). Lambrakis, protected by his parliamentary immunity, marched alone and arrived at the end of the rally holding the banner with the peace symbol , the one he had previously held up during the Aldermaston rally in the United Kingdom while protesting near the Atomic Weapons Research Establishment (AWRE). Soon afterward, he too was arrested by the police.
On May 22, 1963, after delivering the keynote speech at a pacifist meeting in Thessaloniki, he was run down by a delivery truck driven by two right-wing extremists (Emmanouil Emmanouilidis and Spyros Kotzamanis). He suffered brain injuries and died in the hospital five days later, on May 27.
The next day, in Athens, his funeral became a massive demonstration. More than 500,000 people rallied to protest against the right-wing government and the Royal Court, seen by many to support the activities of the right-wing extremists. The assassination of Lambrakis initiated an enormous popular reaction, and soon after, Investigator Christos Sartzetakis (who later in the 1980s was elected as President of the Republic) and Attorney General P. Delaportas uncovered the government's connections to right-wing extremists; not surprisingly, both men soon lost their jobs.
The events that followed the assassination of Lambrakis lead to rapid political developments. Prime Minister Constantine Karamanlis resigned and fled to Paris in July 1963. The Marathon Peace Rally became an annual event in Lambrakis's memory. Thousands of Greek youths formed a new political organisation called The Neolaia Lambraki (Νεολαία Λαμπράκη - Lambrakis Youth); the first secretary of Neolaia Lambraki was Mikis Theodorakis, one of Lambrakis's friends and fellow activists. This liberal political organisation played a decisive role in Greece's left-wing democratic movement of the 1960s.
The life and death of Gregoris Lambrakis inspired the author Vassilis Vassilikos to write the political novel "Z". The title stands for the first letter of the Greek word "Zei" ("He is alive!"), a popular slogan which began to appear on the walls of the buildings of the Greek cities in the 1960s, illustrating the growing protest against the conditions that led to the assassination of Lambrakis. In 1969, the Greek film director Costa-Gavras (Κώστας Γαβράς) made the film Z, which was a great success. Yves Montand starred as Lambrakis, Jean-Louis Trintignant as investigator Sartzetakis and Irene Papas as Lambrakis's widow.
Lambrakis remains in the hearts of the Greek people as a national symbol of democracy, representing the struggle against political repression, Royal Court scandal, and international dependence. After the fall of Junta in 1974, numerous places, including a football stadium in Kallithea and streets and squares throughout the country, have been named in honor of Gregoris Lambrakis.