Porous boron nitride (BN) has proven promising as a novel class of inorganic materials in the field of separations and particularly adsorption. Owing to its high surface area and thermal stability, porous BN has been researched for CO2 capture and water cleaning, for instance. However, research remains at the laboratory scale due to a lack of understanding of the formation mechanism of porous BN, which is largely a “black box” and prevents scale up. Partial reaction pathways have been unveiled, but they omit critical steps in the formation, including the porosity development, which is key to adsorption. To unlock the potential of porous BN at a larger scale, we have investigated its formation from the perspective of both chemical formation and porosity development. We have characterized reaction intermediates obtained at different temperatures with a range of analytical and spectroscopic tools. Using these analyses, we propose a mechanism highlighting the key stages of BN formation, including intermediates and gaseous species formed in the process. We identified the crucial formation of nonporous carbon nitride to form porous BN with release of porogens, such as CO2. This work paves the way for the use of porous BN at an industrial level for gas and liquid separations.