Upon returning home, his work among the street people of London cemented his reputation, and is regarded as a classic instance of social documentary which laid the foundations for photojournalism.

In 1861, he became a member of the Royal Scottish Society of Arts, but by 1862 he had decided to travel to Singapore to join his older brother William, a watchmaker and photographer.

After visiting Ceylon and India from October to November 1864 to document the destruction caused by a recent cyclone, Thomson sold his Singapore studio and moved to Siam.

After arrival in Bangkok in September 1865, Thomson undertook a series of photographs of the King of Siam and other senior members of the royal court and government.

Inspired by Henri Mouhot's account of the rediscovery of the ancient cities of Angkor in the Cambodian jungle, Thomson embarked on what would become the first of his major photographic expeditions.

He set off in January 1866 with his translator H. G. Kennedy, a British Consular official in Bangkok, who saved Thomson's life when he contracted jungle fever en route.

Thomson then moved on to Phnom Penh and took photographs of the King of Cambodia and other members of the Cambodian royal family, before travelling on to Saigon.

He established a studio in the Commercial Bank building, and spent the next four years photographing the people of China and recording the diversity of Chinese culture.

His subject matter varied enormously: from humble beggars and street people to Mandarins, Princes and senior government officials; from remote monasteries to Imperial Palaces; from simple rural villages to magnificent landscapes.

The project documented in photographs and text the lives of the street people of London, establishing social documentary photography as an early type of photojournalism.

In 1881 he was appointed photographer to the British royal family by Queen Victoria, and his later work concentrated on studio portraiture of the rich and famous of "high society", giving him a comfortable living.

After retiring from his commercial studio in 1910, Thomson spent most of his time back in Edinburgh, although he continued to write papers for the Royal Geographical Society on the uses of photography.

Evidence to Phiphat's claims include an analysis of a photograph in which the temple Wat Ratchapradit, which was built before Thomson arrived in Bangkok, is missing.