He worked in various government ministries, statutory boards, and agencies, helping to shape public policies and national projects, leading to Singapore's economic growth in the first few decades after independence.

The newly created division only consisted of three other people — Minister for Finance Goh Keng Swee, permanent secretary Hon Sui Sen, and the office boy.

The plan aimed to increase employment opportunities in Singapore through a series of investments from the World Bank and the United Kingdom.

[2][5] In 1968, Ngiam, together with Goh Keng Swee, initiated Singapore's first and largest purchase of gold from the Government of South Africa.

[11] On 28 July 1970, Ngiam led a delegation of five to the British Board of Trade, to discuss launching a regular flight service between Singapore and London, operated by Malaysia-Singapore Airlines (MSA).

[19][20][21] On 19 October 1970, Ngiam led another delegation of four to Bern, to discuss launching a regular flight service between Singapore and Rome, operated by MSA.

Ngiam led the Singapore delegation, and discussed the distribution of existing air routes and fleet of aircraft to both airlines.

[33] In addition to his existing portfolio, on 12 February 1979, Ngiam was appointed as permanent secretary in the Prime Minister's Office (PMO).

[35][36] On 1 April 1980, Ngiam succeeded Ridzwan Dzafir as the chairman for the watchdog committee on inflation, ensuring that shops and supermarkets cease from price gouging.

[38][39] However, in December 1980, Minister for Trade and Industry Goh Chok Tong requested Ngiam to remain as chairman for another year, while reassuring that Hwang will be the successor.

[68][69] From 1999 to 2019, Ngiam served as the pro-chancellor for the National University of Singapore (NUS), and was an adjunct lecturer Lee Kuan Yew School of Public Policy from 2005 to 2017.

[70] During a dialogue session held at NUS in 2012, Ngiam urged academics to "help the State to ask the right questions" by offering their opinions on national issues, arguing that this was the key to Singapore achieving economic development.

Stressing that universities should not research for the sake of doing so, he reiterated that academics across disciplines should come together to study a local issue, so that "thinkers in ivory towers" can make better decisions for Singapore.

When asked for his opinions on transforming Singapore into a global city, he questioned the benefit of organising the Formula One (F1) night race yearly, and believed that taxpayer money should be spent wisely and not on a frivolous expense such as F1.

[76] In 2006, NUS Press published the book A Mandarin and the Making of Public Policy: Reflections by Ngiam Tong Dow, providing his opinions and criticisms on the economic foundation laid by the founding generation of political leaders in Singapore.