It is almost two and half decades since the government of Tanzania established the Mafia Island Marine Park. Guided by the principle of participatory resource governance, this significant socio-economic move was expected to promote sustainable marine resource use and conserve ecosystem processes and biodiversity. However, this is contrary to what is befalling today. This qualitative study sought to examine how this cardinal governance rule was applied during the establishment of the park and how the nature of its execution could have a bearing on the current exhaustion and destructive course. The study used focus group discussions, in-depth interviews, life histories, and observations to collect data. Both, manual content analysis and software-aided qualitative data analysis (Nvivo—12) were applied to gain a sense of the data. The findings of the study indicate that the current unsustainable marine resource practices in the park are, in some measure, a result of inconsiderate, poor, and disingenuous participation exercised during and after the inception of the park in 1995. The study recommends a democratically genuine participatory process in which the most affected, collectively or individually, actively decide on the course of action to address their genuine concerns.