Pengiran Abdul Momin began his career as a clerk in 1946 and was appointed an administrative officer in January 1955, becoming the assistant
district officer of
Belait. In August 1958, he was promoted to district officer, a position he held until September 1959. During this time, he was sent to
Ottawa, Canada to take a diploma in political science and public administration at
Carlton University. He returned to the role of district officer of Belait in 1963. A month later, he announced the commencement of an
B$8,500 project in the Belait district, which included constructing small roads—one between
Lumut and
Sungai Liang spanning and another between
Bukit Sawat and Merangking Ulu measuring 700 feet—as well as cleaning Sungai Terima up to Simpang Labit. Later that year, on 28 July, he officiated the opening of the adult Malay language class in Lumut and commended the villagers for supporting the government's literacy initiative. In August, Pengiran Abdul Momin also officiated the opening of the adult religious class for men at Ahmad Tajuddin Malay School in
Kuala Belait. Pengiran Abdul Momin and
Abidin Abdul Rashid attended the UN conference on human rights for developing nations from 12 to 18 May 1964 in
Kabul, Afghanistan, as delegates of Brunei. In July, he announced that five villages in the Belait District—Sungai Liang, Lumut,
Kuala Balai,
Labi, and Bukit Sawat—would soon have community halls, funded with $40,000 under the Rural District Development Plan, to host social gatherings, meetings, and welfare activities. Later that year, on 21 November, he officiated the opening of
Melilas Malay School. During the third Belait District Council meeting on 5 June 1965, Pengiran Abdul Momin announced the construction of a rest house in Labi to accommodate government officers on official duties in the area. He also outlined plans for three community halls in
Sungai Teraban,
Sukang, and Melilas, along with additional rest houses in Bukit Sawat and Melilas. A community hall and another rest house were also scheduled for construction in Kuala Belait. In addition, a road linking Sukang to Bukit Sawat was set for immediate construction, with completion expected within the year. As chairman of the Belait District Council meeting in December 1966, Pengiran Abdul Momin provided clarification on the efforts undertaken by the local agricultural office in response to concerns about the Batu 3 experimental farm in
Seria. Later, in October 1967, he accompanied the acting chief minister,
Pengiran Muhammad Yusuf, on a visit to development projects in Kuala Belait. During the visit, they inspected the proposed site for a
new $13 million hospital to replace the existing one, along with proposed sites for a vocational school, a secondary school, a turbine gas power plant, a community hall, and two relocation projects. In 1969, Pengiran Abdul Momin was transferred to the
State Secretary's Office to serve as acting assistant state secretary of Brunei. Pengiran Abdul Momin was appointed as state secretary of Brunei on 25 March 1970, with the sultan's approval, He was appointed as acting chief minister of Brunei from 16 to 31 August, while the incumbent, Pengiran Muhammad Yusuf, was on an official visit to Japan, and again served as acting chief minister from 5 November 1970 until the latter's return to Brunei. His tenure as state secretary concluded on 15 May 1972. The declaration and presentation ceremony for his commissioning letter as chief minister was held on 26 April 1975. On 19 November 1976, he clarified that Bank Darussalam had not been licensed to operate in the country, despite claims in the press, and also welcomed the opening of Kalimantan Finance and Investment Berhad's new branch in
Bangar as the area's first financial centre. He completed his tenure as chief minister on 31 August 1981. == Diplomatic career ==