Educated at University College London and called to the bar at Lincoln's Inn, he practised for a few years in a law firm before beginning a career with the Singapore Legal Service, serving with the Attorney-General's Chambers as Crown Counsel and deputy public prosecutor (1960–1963), and senior state counsel (1966–1967); with the Subordinate Courts as a magistrate (1956–1959), first district judge (1967–1970), and senior district judge (1971–1978); and with the Supreme Court as deputy registrar and sheriff (1959–1960), and registrar (1963–1966).

In 1978 he was elevated to the office of Judge of the High Court of Singapore, and served until his retirement in 1997. Notable cases judged by Sinnathuray included the Toa Payoh ritual murders trial in 1983, the 1988 legal challenge by the Asian Wall Street Journal against the Government's move to restrict its circulation for having engaged in the domestic politics of Singapore, and the trial of the serial murderer John Martin Scripps in 1995.

Following his retirement from the bench he pursued his interest in numismatics, becoming the chairman and chief executive officer of Mavin International Pte.

He continued his secondary education in Raffles Institution after World War II, completing his Senior Cambridge examinations there in 1948.

Between 1960 and 1963 he was a Crown Counsel and deputy public prosecutor with the Attorney-General's Chambers (AGC), before returning to the supreme court to become the registrar from 1963 to 1966.

[8] Three weeks before his retirement from the Bench, on 29 August 1997, Sinnathuray made legal history when he ordered a 29-year-old serial sex offender Kelvin Lim Hock Hin to serve 40 years in prison for sexually abusing five boys in view of his paedophilic condition and high propensity to re-offend, and Lim's sentence was the reportedly the longest term of imprisonment imposed in a sexual crime back in 1997.

[11] Sinnathuray also served as the third president of the Military Court of Appeal from 1990 until 30 November 1997, and was succeeded by Justice Goh Joon Seng.

[12] On 16 May 1988, Sinnathuray ruled that the Government was entitled under the Newspaper and Printing Presses Act[13] to restrict the circulation of the Asian Wall Street Journal for having engaged in the domestic politics of Singapore.

B. Jeyaretnam that the Government had interfered with the subordinate judiciary by ordering the transfer of Senior District Judge Michael Khoo to the AGC as Senior State Counsel and Deputy Public Prosecutor after Khoo had rendered a judgment which had allegedly displeased the Government.

[21] In 1993, the Singapore Chapter of University College London Alumni was founded by Sinnathuray (who served as its first president) and his fellow alumni friends to provide for the welfare and interest of UCL graduates in Singapore and as a local platform for networking with members from other chapters around the world.

[21] He was survived by his wife Sandra Devi; his son Chandra Raj, daughter Shamona Ranee, and their spouses; and three grandchildren.