Initially in private legal practice in Penang after being called to the Bar there, the Director of the Legal Aid Bureau (1959), In 1961, he represented Singapore at the Seminar on the Protection of Human Rights in the Administration of Criminal Justice organised by the United Nations in
Wellington, New Zealand. With effect from 1 September 1963, he became the Solicitor-General, succeeding T. Kulasekaram who had been appointed a Supreme Court judge. which involved four executives of the
Nanyang Siang Pau (
South Seas Business Newspaper), a Chinese-language newspaper, who had been detained without trial under the
Internal Security Act ("ISA") for "glamorising communism and stirring up communal and chauvinistic sentiments over Chinese language, education and culture". Upon an application by the detainees for
habeas corpus, the High Court held that the
President, when acting on
Cabinet's advice to detain a person under the ISA, exercises a subjective discretion as to whether the person is a risk to
national security, which is not
justiciable by the courts. Although the judgment was later disapproved by the
Court of Appeal in
Chng Suan Tze v. Minister for Home Affairs (1988), in 1989
Parliament amended the
Constitution and the ISA to "freeze" the law relating to detentions under the Act to that applying in Singapore on the date when
Lee Mau Seng was decided. In the 1970s, the financial collapse of two companies, Gemini Chit Fund Corporation and Stallion Corporation, led to criminal charges being brought against their executives. The companies operated
chit funds, which were schemes in which investors paid sums of money by instalments to the companies in exchange for the entitlement to be allotted a larger return from the fund. The allotment was determined by lot or by auction. Following their collapse, it was estimated that 40,000 investors had lost about
S$50 million. and its chairman V.K.S. Narayanan received nine months' imprisonment for two charges under the Companies Act. Stallion's executive director Martin Ler Cheng Seng pleaded guilty to authorising his firm to unlawfully bid at a Stallion chit fund sale, and was jailed for a year. Tan also successfully brought proceedings for
scandalising the court against
Wong Hong Toy, the Chairman of the
Workers' Party of Singapore, in 1983, and against respondents involved in publishing, printing and distributing articles that appeared in the
Asian Wall Street Journal in 1985 and 1991. Where
public international law matters were concerned, in 1966 Tan attended the Sixth Committee on International Law at the 21st Session of the
United Nations General Assembly in New York, and chaired a meeting of law ministers to review extradition arrangements among
Commonwealth nations at
Marlborough House, London, in 1982. He also regularly attended Commonwealth Law Ministers' Conferences and
ASEAN Law Association Conferences. and revised editions of
Singapore statutes in 1970 and 1985. A 1990 revised edition of
subsidiary legislation was subsequently published in 1992. In January 1990, the Attorney-General's Chambers launched LawNet, a computer database then containing the full text of
Singapore legislation, at the cost of $4.3 million. Tan was involved in the establishment of the
Singapore Academy of Law, a professional association of judges, lawyers, Legal Service officers and law academics, and served as its vice-president from 1992. and held the post until August 1999. In March 1992, he appointed a committee to review Singapore's arbitration laws, which then dealt only with domestic
arbitration, to bring them "in line with international developments". The committee's work led to the enactment of the International Arbitration Act in 1994. In addition to his Attorney-General post, Tan was the Chairman of the
Singapore Petroleum Company (SPC; 1971 – August 1999), a director and deputy chairman (from 1985) and later chairman (June 1990 – 1994) of the Insurance Corporation of Singapore (ICS), Vice-president (from 1972) and President (from 1992) of the Singapore Musical Society, and a fellow of the Singapore Institute of Directors. ==Later years==