The museum began in October 2013 in a building on David Street in South Dunedin called the Legalise Cannabis House. Gray, then deputy leader of the Aotearoa Legalise Cannabis Party, and Julian Crawford, the party's spokesperson, created the space as a joint venture with Otago NORML. Dunedin had a long history of marijuana law reform advocacy, and the museum served as an information center stocking books unavailable in conventional libraries. It was deliberately educational rather than commercial -- Whakamana operates within New Zealand's laws and does not sell cannabis. Gray envisioned the museum positioning Dunedin as a future hub for cannabis tourism, should legalization ever arrive. In 2017, he left the ALCP to support The Opportunities Party, believing it offered a more viable path to reform. The party's leader, economist Gareth Morgan, welcomed the defection.

The Dunedin museum closed in 2018 after Gray moved to Christchurch and could not find volunteers to keep it running. The following year brought an unlikely partnership: Michael Mayell, the founder of Cookie Time -- one of New Zealand's best-known snack brands -- saw the museum as a vehicle for his interest in hemp foods and sustainability. Mayell argued that a hemp economy could help New Zealand reduce its dependence on dairy farming. Together, Gray and Mayell reopened the museum in Shand's Emporium, a heritage building on Manchester Street in Christchurch that had been restored after the 2011 earthquake. They planned to expand into the adjoining Trinity Church, one of Christchurch's oldest buildings. Briefly, the museum operated New Zealand's only Cannabis Social Club and hosted talks by activists and medicinal cannabis advocates, including Rose Renton.

To complete the Trinity Church restoration and expand their operations, Gray and Mayell launched a crowdfunding campaign on PledgeMe in late 2019, seeking NZ$1 million. By the close, 299 people had pledged $214,616 -- impressive enthusiasm, but far short of the minimum target. Under PledgeMe's rules, every dollar was returned. Gray blamed a lack of awareness within the cannabis community, then later pointed to the stigma that still surrounded cannabis in New Zealand. The timing was cruel: the 2020 cannabis referendum was approaching, and Gray and Mayell had planned to develop Whakamana into a social club and dispensary depending on the outcome. The referendum narrowly failed, with 50.7 percent voting against legalization. The Christchurch location closed, and Gray moved to Wellington for yet another attempt -- scuppered almost immediately by COVID-19.

Auckland, the country's largest city, offered the critical mass of supporters and foot traffic that Dunedin, Christchurch, and Wellington could not. In 2023, Gray launched another fundraiser, this time targeting the Hopetoun Alpha building -- a heritage property in Auckland's central business district. An unnamed wealthy donor contributed NZ$500,000, finally providing the financial stability that had eluded the project for a decade. On 9 August 2024, MP Chloe Swarbrick officially reopened Whakamana in a ribbon-cutting ceremony. The building, dubbed the "Green House," now shares space with Calyx Clinic, a legal medicinal cannabis dispensary, and hosts a patient social club. The museum continues its original mission: education, destigmatization, and a comprehensive collection documenting the science, history, and legislation of cannabis in New Zealand and beyond.