Williams was first elected to the Cook Islands parliament as a Cook Islands Party candidate for the electorate of Aitutaki at the 1968 election. He served as Minister of Health and Education from 1974 to 1978 in the government of Albert Henry, as well as being Henry's personal physician. As Health Minister, he supported Czech cancer-therapist Milan Brych relocating his clinic to the Cook Islands, despite Brych being removed from the medical register in New Zealand. He was one of three prominent CIP members to leave the party immediately before the 1978 election, and subsequently contested the Arorangi electorate for the Unity Party, but lost his seat. Williams subsequently migrated to New Zealand. He rejoined the Cook Islands Party and was re-elected in the 1994 election as its candidate for the Overseas seat, representing Cook Islanders living abroad. He served as Minister of Health, Tourism, Transport and State-Owned Enterprises from 1994 to 1996. Although a member of the Cook Islands Party, he opposed the premiership of Geoffrey Henry, the party's leader from 1979 to 2006. Many members of the Cook Islands Party opposed its coalition agreement with the New Alliance Party, and after several defections, Henry lost control to the dissidents and resigned. In July 1999, Williams narrowly won endorsement as the new Prime Minister. This prompted considerable anger in some quarters, primarily because Williams mostly lived outside the islands. In October 1999 the New Alliance Party left the governing coalition, depriving the government of its majority. Williams attempted to form a new government, but the following month he lost a vote of no-confidence to the oppositionDemocratic Alliance Party and the New Alliance Party. Terepai Maoate became the new Prime Minister. He lost his seat when the overseas electorate was abolished in 2003. Williams later attempted to start a political career in the New Zealand context, standing as a candidate for the New Zealand First party at 15th place in the party list and contesting the Maungakiekie electorate seat in the 2005 parliamentary elections. He did not succeed in becoming an MP through either poll. Williams re-entered New Zealand politics before the 2017 New Zealand general election, founding the One Pacific Movement. One Pacific later reached a deal with the Māori Party under which it ran Pasifika candidates on the Māori Party list.
After politics
Williams served as a member of the World Health Organization's executive board from 1995 to 1997. In 1999, Williams was found to have breached ethical standards by New Zealand's Health and Disability commissioner after one of his patients was misdiagnosed and given tablets that were past their expiry date. In 2002, Williams became involved in a controversy regarding a proposed medical trial in the Cook Islands. 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. Later, the Cook Islands nominated Williams to head the World Health Organization. In 2018 Williams was fined NZ$10,000 plus NZ$145,000 in court costs after using an unapproved "magic cream" to treat eczema. He was awarded the Queen’s Service medal in 1974, and the Pasifika Medical Association Service Award in 2004. He was made a Companion of the Queen's Service Order for services to the Cook Islands community in the 2011 New years Honours list.
