Manus Island is covered in rugged jungles which can be broadly described as lowland tropical rain forest. The highest point on Manus Island is Mt. Dremsel 718 metres above sea level at the centre of the south coast. Manus Island is volcanic in origin and probably broke through the ocean's surface in the late Miocene, 8–10 million years ago. The substrate of the island is either directly volcanic or from uplifted coral limestone.
Manus Island is home to the Emerald green snail, whose shells are harvested to be sold as jewellery.
The first recorded sighting of Manus Island by Europeans was by Spanish explorer Álvaro de Saavedra on board the carrack Florida on 15 August 1528, while trying to return to New Spain from the Maluku Islands. Saavedra circled Manus Island, and landed possibly on Murai islet to the south west. Murai was found to be inhabited and some natives came out in canoes, attacking with bows and arrows. Three of these men were captured by the Spaniards, and were returned by Saavedra to the same island, on his second attempt to return to North America the following year. Manus Island was charted as Urays la Grande or Big Urays, which is probably a projection of Murai to signify "big Murai".
In World War II Manus Island was the site of an observation post manned by No. 4 Section, 'B' Platoon, 1st Independent Company, Australian Imperial Force, who also provided medical treatment to the inhabitants. Manus was first bombed by the Japanese on 25 January 1942, the radio mast being the main target. On 8 April 1942 an Imperial Japanese force consisting of the light cruiser Tatsuta, destroyer Mutsuki and a troop transport ship Mishima Maru entered Lorengau harbour and several hundred Japanese soldiers of the 8th Special Base Force swarmed ashore onto the Australian-mandated island. The vastly outnumbered Australians withdrew into the jungle.
Later in 1942, Japan established a military base on Manus Island. This was attacked by United States forces in the Admiralty Islands campaign of February–March 1944. An Allied naval base was established at Seeadler Harbor on the island and it later supported the British Pacific Fleet.
In 1950–51 the Australian government conducted the last trials against Japanese war criminals on the island. One case heard was that of Takuma Nishimura, who faced an Australian military court. He had already been tried by a British military court in relation to the Sook Ching massacre in Singapore and sentenced to life imprisonment. While on a stopover in Hong Kong he was intercepted by Australian Military Police. Evidence was presented stating that Nishimura had ordered the shootings of wounded Australian and Indian soldiers at Parit Sulong and the disposal of bodies so that there was no trace of evidence. In this trial he was found guilty and was hanged on 11 June 1951.
American anthropologist Margaret Mead lived on Manus Island before and after the war, and gave detailed accounts in Growing up in New Guinea and New Lives for Old.
Australia set up the Manus Regional Processing Centre on Manus Island in 2001 as part of its Pacific Solution. The last inmate in that period was Aladdin Sisalem, who was kept as a lone inmate from July 2003 until he was finally granted asylum in Australia in June 2004. In August 2012, the Australian Government controversially announced it would resume offshore processing; in November 2012 the relocation of asylum-seekers to Manus Island resumed.
The Australian government's decision to resume offshore processing has met with domestic political opposition from the Greens, and a controversial decision by the Australian government in July 2015 to make reporting of abuse within the centre illegal prompted staff at those centres to begin a campaign of civil disobedience.
On April 26, 2016, the Supreme Court of Papua New Guinea ruled that the detention of asylum seekers on Manus Island was illegal.
Australia's immigration minister, Peter Dutton, confirmed on 17 August 2016 that the centre was to be closed, but no timescale was given. The Manus Detention Centre closed on 31 October 2017.