Malalasekera was appointed the first Ambassador for Ceylon to the USSR in 1957 by Prime Minister S. W. R. D. Bandaranaike when he established diplomatic relations with socialist countries such as Russia, China, Czechoslovakia, Yugoslavia. In 1959, he was appointed concurrently first Ambassador for Ceylon to Czechoslovakia, Poland and Romania.[3]
Subsequently, he functioned as the Ceylon High Commissioner in Canada and was Ceylon UN Permanent Representative in New York. There he served as chairman, Security Council Member, Fact Finding Mission to Saigon and also in the Committee on Information from North Non-self Governing Territories. Finally, he was the Ceylon's High Commissioner in the UK from 1963 to 1967.
In 1967, he returned to the island to accept the post of chairman of the National Higher Education Commission which responsible post he held till 1971. He died on 23 April 1973.