He contested the 1989 general elections in the Kalutara electorate from the Sri Lanka Freedom Party and was re-elected to parliament. In 1991, he chaired the Parliamentary select committee on the ethnic conflict (known as the "Mangala Moonesinghe Committee"), gaining consensus among the Sinhala and Tamil members of parliament in the committee. He contested the 1994 general elections, but failed to gain enough preferential votes to be re-elected to parliament, although his party won the election and formed a new government. The Sri Lanka Freedom Party government of Chandrika Kumaratunga appointed Moonesinghe as Sri Lankan High Commissioner to India in 1995 and he served till 2000, when he was appointed Sri Lankan High Commissioner to the United Kingdom, where he served until 2002, when the Sri Lanka Freedom Party government was defeated in the 2001 general elections. Following his return from the United Kingdom; he served as chair of the "One Text Initiative", Chairman of Marga Institute and was a director of the Carson Cumberbatch & Co. He died in 2016 at the age of 84. Moonesinghe married Gnana Coomaraswamy. They had a son and a daughter. They lived at Park Road, Battaramulla, before settling at Havelock City. Dr Kumaran Ratnam was his brother-in-law.