He principally resided in Auckland, New Zealand, where he was medical director of the Mt Wellington Integrated Family Health Centre.

[3][5][6] Williams graduated from Otago Medical School in 1960, and later completed a Masters in Public Health at the University of Hawaiʻi.

[5] He returned to the Cook Islands in 1964, where he worked as Medical Superintendent, surgeon, physician, Director of Health and Secretary of Social Services, while also researching tropical diseases, including filariasis.

[7] In 2002 the Cook Islands Government unsuccessfully nominated Williams to head the World Health Organization.

[10] In 1999, Williams was reportedly found by New Zealand's Health and Disability Commissioner to have breached ethical standards when one of his patients was misdiagnosed and given tablets that were past their expiry date.

[12] The trial, which involved injecting pig cells into humans as a means of fighting diabetes, could not legally take place in New Zealand, and therefore moved to the Cook Islands, where less strict regulations applied.

Williams, a strong supporter of the proposal, believed that it would bring benefits to the Cook Islands' economy, but the scheme aroused much controversy.

[32] In 2015, Williams was named inaugural Patron of the Pasifika Medical Association [33] and in 2016 he received the World Health Organization's Award of Appreciation for his contribution to eliminating lymphatic filariasis.