The previously undescribed population dynamics of the Western Australian soldier crab, Mictyris occidentalis Unno, 2008, in King Bay, north-western Australia are elucidated, with annual observations and sampling over a 30-year period from 1980 to 2010. This is the longest recorded study of a single inter-tidal brachyuran population and shows long-term persistence of soldier crab populations in stable, sheltered habitats. The life cycle of M. occidentalis follows a cryptic subsurface juvenile and immature adult (= adolescent) phase and an emergent adult phase. Population densities were generally 500 crabs/m2 for the early 1980s, late 1980s, and during the early 2000s. Maximum population densities were high in the mid 1980s (800 crabs/m2). The spatial and temporal variability in the distribution of the population was consistent over the study period. Juvenile recruitment extends for up to 7 months of a given year between May and November with the main influx of juveniles usually in June and occasional minor influxes in August or October. Juvenile recruitment is followed by incremental growth of individuals at a rate of 1 mm/month reaching sexual maturity in the first year at 6.0-6.9 mm carapace length (CL). Adult males are larger than females with a maximum size of 15.0 mm CL compared to 12.0 mm CL, respectively. Ovigerous females are low in numbers throughout most of the year but reach a peak in February. During swarming, M. occidentalis populations partition not only by size class, with surface crabs being adults only and subsurface crabs mainly adult females and juveniles, but also by sex in that swarms are male-dominated in varying ratios.