We previously observed that long-horizontal lightning flashes exceeding 100 km in length, known as “megaflashes,” occur preferentially in certain thunderstorms. In this study, we develop a cluster feature approach for automatically documenting the evolutions of thunderstorm systems from continuous lightning observations provided by the Geostationary Lightning Mapper (GLM) on NOAA’s Geostationary Operational Environmental Satellites (GOES). We apply this methodology to GOES-16 GLM observations from 2018 to mid-2022 to improve our understanding of megaflash-producing storms. We find that megaflashes occur in long-lived (median: 14 hours) storms that grow to exceptional sizes (median: 11,984 km2) while they propagate across long distances (622 km) compared to ordinary storms. The first megaflashes are typically produced within 15 minutes of the storm reaching its peak intensity and extent, describing the transition to mature convection. Most megaflashes occur 13 hours after the initial megaflash activity, and are sufficiently close to convection to suggest initiation in the convective line (where GLM has difficulty detecting faint early light sources from these megaflashes). In-situ generated megaflashes are rare, accounting for 2.7% of the sample using a 50 km convective distance threshold, but also tend to larger than normal megaflashes, possibly due to having direct access to the electrified stratiform cloud through which megaflashes propagate.