He was seconded in 1979 to the Pacific Islands Forum Secretariat, before being posted to Japan in 1980 as Chargé d'Affaires, entrusted with the task of establishing Fiji's Embassy in Tokyo.

In 1987, he served at Fiji's Government House as Permanent Secretary to Governor-General Ratu Sir Penaia Ganilau.

After the September 1987 coup d'état, he "found himself a target as the high-profile white permanent secretary to Fiji's governor-general, embroiled in a constitutional crisis and with indigenous supremacists demanding his head".

"[15] His New Zealand-registered company, Thomson Pacific, managed Mitsubishi Trust Bank's real estate assets in Auckland.

A few months before his appointment, Thomson had publicly criticised what he described as Australia's "ongoing campaign to choke off Fiji’s role as an international peacekeeper".

In 2012, he successfully led the campaign for Fiji's election to Chairmanship of the Group of 77 and China, the UN's largest negotiating bloc of 133 developing countries, and served in that capacity throughout 2013.

In his capacity as Fiji's Permanent Representative, Ambassador Thomson undertook the UNDP Presidency role, with Executive Board oversight responsibilities for over US$7 billion of UN annual funding.

Held at the UN campus in June 2017, it was co-hosted by Fiji and Sweden, and is widely regarded as the turning point in global action to reverse the decline in the Ocean's health.

In October 2017 he was appointed as the United Nations Secretary-General's Special Envoy for the Ocean, in which role he leads UN advocacy and outreach efforts to galvanise political momentum, mobilise action and raise ambition for the implementation of the UN's Sustainable Development Goal 14.

[25] He is the editor and publisher of the pictorial/historical book Fiji in the Forties and Fifties, written by his father, with photographs by Rob Wright,[26] and is the author of Wild Vanilla.