Catalytic methane decomposition (CMD) is an environmentally friendly method of hydrogen production that, unlike other conventional processes, such as steam methane reforming, partial oxidation of methane, and dry reforming of methane, can convert methane into hydrogen with a simultaneous generation of solid carbon without CO₂ emissions. This study mainly focused on the application of carbon-based catalysts derived from biomass and biowaste for the CMD process. For this purpose, eight catalysts were produced from three carbon materials (wood, sewage sludge, and digestate) through the subsequent processes of pyrolysis, leaching, and physical activation. The comparison of catalysts prepared from the slow pyrolysis of biowaste and wood indicated that carbon materials with a lower ash content achieved a higher initial methane conversion (wood char > digestate char > sewage sludge char). For feedstocks with a high initial ash content, such as digestate and sewage sludge chars, an improvement in the catalytic activity was observed after ash removal through the leaching process with HNO₃. In addition, physical activation through CO₂ fluxing led to an enhancement in the BET surface area of these catalysts, and consequently to a growth in methane conversion. The initial methane conversion was assessed for all chars under operating conditions of 900 °C, a gas hourly space velocity (GHSV) of 3 L/g/h, and a CH₄:N₂ ratio of 1:9, and it was 65.9, 59.1, and 42.6% v/v, respectively, for chars derived from wood, sewage sludge, and digestate; these values increased to almost 80% v/v when these chars were upgraded by chemical leaching and physical activation.