Marsters served as Minister of Works in the Cabinet of Geoffrey Henry, but resigned his position in 1997 in protest at budget cuts. He later served as Minister of Works, Youth, Sport and Recreation in the first coalition Cabinet of Robert Woonton from 2002, but was sacked in 2003 after a coalition realignment. He rejoined Cabinet after the 2004 election, when Woonton was trying to put together a new coalition; when Woonton resigned to fight a by-election, he served in the Cabinet of Jim Marurai, holding the portfolios of foreign affairs, transport, and youth and sport. In August 2005, Marurai sacked Cook Islands Party leader Geoffrey Henry from Cabinet, causing the CIP to reconsider its role in government. A month later, Marsters was also sacked, and the coalition formally dissolved.
Opposition
The retirement of Geoffrey Henry in 2006 led to a leadership election, which saw Marsters replaced as Deputy leader by Tupou Faireka. However, both Faireka and party leader Henry Puna lost their seats at the 2006 election. While Puna continued to serve as leader outside parliament, Marsters became leader of the opposition.
Marsters was re-elected at the 2010 elections, which saw the Cook Islands Party win 16 of the 24 seats. henry Puna was elected Prime Minister, and Marsters was appointed to Cabinet as Deputy Prime Minister, Minister of Foreign Affairs, and Minister of Mining and Natural Resources. As Minister of Mining and Natural Resources he promoted seabed mining, and negotiated a Regional Seabed Mining Framework through the Pacific Islands Forum. he began negotiations with the Cook Island's neighbours over marine boundaries, and sent a delegation to the United Nations in New York to negotiate an extension of the country's continental shelf boundaries under the United Nations Convention on the Law of the Sea. In June 2011 his son Giovanni Marsters was arrested on 14 charges of importing, selling, and possessing illicit drugs. He later pled guilty and was sentenced to six years imprisonment for importing cannabis.
Queen's Representative
On 5 June 2013 Marsters was appointed Queen's Representative, replacing Frederick Tutu Goodwin. He resigned from Parliament on 25 July 2013 to take up the role, precipitating the 2013 Murienua by-election. In June 2016 he made a rare intervention as Queen's Representative, ruling that Parliament had been properly adjourned and that therefore a purported opposition vote of no-confidence ousting the government was of no effect. He was reappointed for a second three-year term in July 2016, and a third one in 2019. In the 2018 Birthday Honours Marsters was appointed a Knight Commander of the Order of the British Empire. In November 2018 he was knighted in a ceremony at Buckingham Palace. In January 2019 Marsters helped turn the first sod and lay the first stones of Marsters House, a hostel for the various branches of the Marsters family in Rarotonga. The hostel was opened in July 2019.
