Kadirgamar went on to study law at the University of Ceylon from which he graduated with a Bachelor of Laws (LLB) (Honors) degree in 1953 and became the top student in the First Class at the Advocates Intermediate Examination in 1953. In1954 he won the scholarship for the candidate placed first in the First Class at the Advocates Final Examination of the Ceylon Law College and was awarded prizes for the Law of Evidence and the Law of Persons and Property. He was admitted to the Ceylon Bar in 1955. Later, at Balliol College University of Oxford, he received his B.Litt. degree. Kadirgamar, like several other Sri Lankans, was elected to serve as President of the Oxford Union.
Kadirgamar was the author of a number of scholarly articles published in international legal journals such as the Modern Law Review, The South African Law Journal and The Conveyancer and Property Lawyer, etc. An ardent international civil servant, Kadiragamar, held many positions in numerous international organizations including the ILO, World Intellectual Property Organization (WIPO), Indian Ocean Rim Association for Regional Cooperation (IOR-ARC) and South Asia Foundation (SAF). In 1963, he was the Special Commissioner of Amnesty International to investigate Buddhist-Catholic clashes in Vietnam. He was a President's Counsel.
On 20 November 2003, Kadirgamar declared his candidacy for the position of Secretary-General of the Commonwealth of Nations. He stood with the support of South Africa, which was critical of incumbent Don McKinnon's opposition to Zimbabwean involvement in the 2003 Commonwealth Heads of Government Meeting, at which Zimbabwe withdrew from the organisation. In doing so, Kadirgamar broke the convention of not challenging incumbents for the position, which is usually appointed by consensus. However, at the vote, held in early December, he was easily defeated by the New Zealander, with only 11 members voting for him against 40 for McKinnon.
Following the victory of the United People's Freedom Alliance in the April 2, 2004 Sri Lankan legislative elections, he was mentioned as a candidate for Prime Minister of Sri Lanka, but on April 6 President Kumaratunga appointed Mahinda Rajapaksa to the post. Four days later, however, he became foreign minister again in the new cabinet.
It is widely believed around the world that Kadirgamar's assassins belong to LTTE, a group banned as a Terrorist Organization by a number of countries including the United States, Canada, India and the European Union. A overwhelming majority of Sri Lankans, who are Sinhalese, hold them responsible due to the fact that Kadirgamar was responsible for getting the US among many countries to classify the LTTE as a terrorist organization. The Tamil Tigers have denied responsibility for the killing, and have stated that any damage made to their international reputation by Lakshman Kadirgamar was already done, and they would not risk the cease-fire agreement by carrying out this assassination. This was seen merely as an expected statement from the LTTE after it had carried out a deadly attack and was not taken seriously by the International community.
According to Asian Tribune, an anti-Rebel website, claimed that On September 5 Sri Lankan police arrested two Tamils, Muttiah Sahadevan alias Devan (Kadirgamar's neighbor's gardener) and Isidor Arokya Nathar alias Babu. The website alleged that they have confessed to having met Charles of the LTTE intelligence and helping out two alleged assassins. The anti-rebel website further claims that this is seen in some quarters in Sri Lanka as proof that the LTTE was behind his assassination.
U.S. Secretary of State Condoleezza Rice, who last met Mr Kadirgamar in June, condemned the assassination of Sri Lanka's foreign minister as a "senseless murder and vicious act of terror" and urged Sri Lankans not to let it lead to resumed civil war. She praised Kadirgamar as a man of "dignity, honor and integrity, who devoted his life to bringing peace to Sri Lanka".
Australia's Foreign Minister Alexander Downer paid tribute to Kadirgamar saying he played a key role in bringing forward the peace process in Sri Lanka. "Lakshman Kadirgamar was an eminent statesman and a distinguished representative of Sri Lanka," he said. "He was a man of moderation who sought the path of peace and worked tirelessly for his country."
Kadirgamar commented later "There was a storm of protest in Australia. A TV interviewer asked me whether I had ever played cricket. I said I had played before he was born - without helmets and thigh guards, on matting wickets that were full of holes and stones, and I had my share of broken bones to show of it. My friend the Australian foreign minister was drawn into the fray. He phoned me. We decided to cool things down... When the whole episode was over I sent a bouquet of flowers to my Australian counterpart. Flowers are also for sissies."