After he emerged from the forests in 1990, he engaged in public activism for rainforest preservation and the human rights of indigenous peoples, especially the Penan, which brought him into conflict with the Malaysian government.

[2][3] At age 19, Manser spent three months in Lucerne prison because, as an ardent follower of non-violent ideologies espoused by Mahatma Gandhi (Satyagraha), he refused to participate in Switzerland's compulsory military service.

After learning more about the tribe, he decided to attempt to live amongst them for a few years and traveled to the East Malaysian state of Sarawak in 1984 on a tourist visa.

After the expedition, he stepped deeper into the interior jungles of Sarawak, intending to find the "deep essence of humanity" and "the people who are still living close to their nature.

[1][7] Manser created notebooks that were richly illustrated with drawings, notes, and 10,000 photographs during his six-year stay from 1984 to 1990 with the Penan people.

[1] In 1988, Manser tried to reach the summit of Bukit Batu Lawi but was unsuccessful, finding himself hanging on a rope without anything to grab on for 24 hours.

As a result, the Penan suffered from reduced vegetation, contaminated drinking water, fewer animals available for hunting, and the desecration of their heritage sites.

[4][8] Manser gave many lectures in Switzerland and abroad, making connections to people within the European Union and the United Nations.

[7] He returned almost every year after leaving the Penan to follow up with the logging activities and to provide assistance to the tribe, often entering these areas illegally, crossing the border with Brunei and Kalimantan, Indonesia.

He discovered that logging conglomerates such as Rimbunan Hijau, Samling, and the WTK Group continued their operations in Sarawak rainforests.

Manser, Kelabit activist Anderson Mutang Urud, and two Penan tribe members travelled from Australia to North America, Europe, and Japan.

[3] After Manser's disappearance, the Federal Assembly finally adopted the Declaration of Timber Products on 1 October 2010, with a transition period allowed until the end 2011.

[3] In 1997, Manser and Christinet tried to enter Peninsular Malaysia from Singapore to fly a motorised hang-glider during the 1998 Commonwealth Games in Kuala Lumpur.

[3] In 1986, Manser's representative in Switzerland, Roger Graf, wrote about sixty letters to Western media outlets, but none took notice of them.

It was only in March 1986 that Rolf Bökemeier, an editor for the GEO magazine based in Hamburg, Germany, who also specialised in indigenous people, wrote a letter of reply to Graf.

Universal Studios started to develop an action-adventure horror script where the Penan used their forest wisdom to save the world from catastrophe.

[9] Warner Bros screenwriter David Franzoni also developed a script named My Friend Bruno after they signed a contract with Manser in January 1992.

[7] Using a forged passport and styling his hair differently,[3] Manser returned to Switzerland in 1990 to inform the public about the situation in Sarawak through the Swiss media.

Sarawak Chief Minister Abdul Taib Mahmud said, "It is hoped that outsiders will not interfere in our internal affairs, especially people like Bruno Manser.

"[18] Sarawak Minister of Housing and Public Health James Wong said that, "We don't want them [the Penan] running around [in jungles] like animals.

"[19] The Sarawak government tightened the entry of foreign environmentalists, journalists, and film crews into the state,[18] and allowed logging companies to hire criminal gangs to subdue the indigenous people.

[15] On 14 November 1986, Manser met with James Ritchie, a reporter from the New Straits Times, in an isolated hut in the Limbang jungles.

On 27 March, Swiss ambassador to Kuala Lumpur Charles Steinhaeuslin made a surprise visit to Sarawak chief minister in Kuching.

[20] In mid-1998, Manser offered an end of hostilities with the Sarawak government if Chief Minister Taib Mahmud would be willing to cooperate with him in building a biosphere around the Penan's territory.

[7] Manser planned to deliver a lamb named "Gumperli" to Taib Mahmud by air as a symbol of reconciliation during the Hari Raya Aidilfitri celebration.

Manser later carried the lamb with him on a plane and parachuted over the United Nations Office at Geneva, hoping to bring international attention to Sarawak.

[7] In March 1999, Manser successfully passed Kuching immigration by disguising himself in a business suit, carrying a briefcase, and wearing a badly knotted tie.

On 29 March, he flew a motorised paraglider, carrying a toy lamb knitted by himself[3] while wearing a T-shirt with the image of a sheep, and made a few turns above Taib Mahmud's residence in Kuching.

"[7] On 15 February 2000, Manser left to visit his Penan friends via the jungle paths of Kalimantan, Indonesia, accompanied by BMF secretary John Kuenzli and a film crew.

They followed Manser's machete cuts into the thick forests until the trail reached the swamp at the foot of Bukit Batu Lawi.