He was the Governor of Madang Province from November 2007 to December 2010, whereupon he resigned the position to be appointed Minister for Justice and Attorney General in Prime Minister Sir Michael Somare's Cabinet. He held these positions until the government was brought down by a parliamentary motion of no confidence in August 2011, whereupon he was relegated to the Opposition benches.
Sir Arnold has also been Chief Justice of Papua New Guinea, Chairman of the Manam Humanitarian Committee, chair of the Commonwealth Observer Group for Solomon Islands' elections, and a member of the international Eminent Persons Group tasked with assessing the political and human rights situation in Fiji following its 2006 military coup.
In April 2011, Amet organised a meeting in Madang so that landowners potentially affected by a proposed Pacific Marine Industrial Zone could express their views and concerns to members of the government. Local MP Ken Fairweather attended, and criticised the project, describing it as a "con-job" which would not benefit his constituents. Fairweather also described Gabriel Kapris (the Minister for Commerce and Industry, who was present) as a stil-man, a Tok Pisin word for a thief. Amet responded by calling Fairweather, who is white, "a white millionaire that the people do not need", telling him: "You pack up and you leave my island. [...] You don't belong here". He added that a white person could never understand the country, and thus could not provide the services that the people needed. (This was disputed by villagers, who pointed out that, under Fairweather, they had benefited from new school facilities, roads, and a greater access to water supplies.) The exchange, which had been filmed, was posted on YouTube, and made front page news in the Post-Courier, the country's largest-selling daily newspaper. The paper described Amet's words as "shamefully wrong and unbecoming of a state minister, especially coming from the former Chief Justice of Papua New Guinea". In response, Amet "took out a full page advertisement" in the paper, in which he apologised to Fairweather and the people of Madang "for [his] unbecoming racial and abusive comments and behaviour".
- "Regional Leaders Making a Difference", Pacific Magazine, 27 April 2008
- "Gau is Madang Governor elect", Post-Courier, 5 January 2011
- "PNG’s Somare again replaces his deputy". www.rnzi.com. Radio New Zealand International. 8 December 2010. Retrieved 15 December 2010. "It is one of a series of cabinet changes that also sees a former chief justice Sir Arnold Amet becoming the attorney general."
- PNG Electoral Commission: National Election 2007, government website
- "Sir Arnold Amet wins seat in election", The National, 4 August 2007
- "Forum Epg To Fiji Completes Consultations", press statement from the Pacific Islands Forum Secretariat, 1 February 2007
- "PNG landowners let Manam people have food gardens". Radio New Zealand International. 10 October 2005. Retrieved 19 September 2011.
- "Madang PMIZ debate turns sour", Post-Courier, 2 May 2011
- "Amet talk upsets islanders", Post-Courier, 3 May 2011
- "Saying sorry is not hard", Post-Courier, 9 May 2011
- Video of part of the exchange between Amet and Fairweather, SoulPNG channel, YouTube, 30 April 2011