The young Weeramantry was very conscious of his country’s heritage and the fact that that while Britain was still a remote province under the Roman conqueror, Sri Lanka had ambassadors at the imperial courts of Rome and China, and that its legal tradition included Buddhist ecclesiastical law, the Roman Dutch law and the common law of Ceylon and England.
His youthful experience of the widely resented British Raj, especially when it used military executions to suppress a minor uprising, and its devious measures to avoid decolonisation in the wake of World War II, could have led him to join one of the rebel movements to dislodge the regime. However the Weeramantry family was not of that mind, and with his brilliance as a student, and following the footsteps of his distinguished civil servant-educator father, he was seen to be a potential future leader.
He gained a BA (Hons) from the University of Ceylon and then a Bachelor of Law from King's College London. Having completed exams at the Colombo Law College he took oaths as an Advocate of the Supreme Court of Ceylon, aged 22.
He was in private practice for 10 years before he and Rosemary de Sampayo married in 1959, and another seven years before being appointed Commissioner of Assize, a post he held for a mere two years before being called to the bench as a Judge of the Supreme Court of Ceylon in1967, aged only 41.
The young dominion was suffering growing pains as it approached the pending change to a republic. Four years later, in April 1971, while in the Jaffna Assize Station, the family of Christopher, Rosemary and five young children was imprisoned in the judge's villa, during the first armed assault on a police headquarters in the insurrection in the northern provincial capital. The conflict was to expose deep economic and social divisions that had been ignored until then and was a forerunner to the subsequent civil war.
By the early 1970s, Weeramantry’s book on the law of contracts had earned the award of a Doctor of Law degree from London University, and his efforts to encourage legal reform had attracted lecturing engagements around the world. Australia's governments, universities, and legal profession were seeking ways to modernise their country's legal system, and Weeramantry was attracted by the opportunity of a university post in Australia.
He was appointed Hayden Starke Professor of Law at Melbourne's relatively new university, Monash, just as Australia was breaking out of the cocoon of the White Australia Policy. There was a marked Eurocentric culture in Australia, and often barriers to Asians holding positions of authority or professional prominence.
Fortunately the university gave him considerable opportunities to research and write, and invitations to lecture internationally and to accept short-term roles overseas continued. These included appointments as senior counsel representing the Government of Fiji in a constitutional case before its Supreme Court; as visiting Law Professor at the University of Papua New Guinea; and as Chair of the Commission of Enquiry into International Responsibility for Phosphate Mining in Nauru.
His experience as Visiting Professor at Stellenbosch University, South Africa, also had far-reaching impact on his determination to work for recognition of human rights. In a rare decision, the university accepted his condition that he would only take up the role if he did not have to accept the status of "Honorary White" to avoid government restrictions on his movement and freedom of speech.
When he returned to Melbourne he wrote Apartheid: The Closing Phases. It was immediately banned in South Africa, but a microfilm copy was smuggled into the country in a fountain pen, copies were produced and circulated, and it had an enormous impact. Several of his other major works were also published during his years at Monash, before he was appointed to the International Court of Justice and left for The Hague in 1990.