Animal personality describes consistent individual behaviours that differ from those of other individuals across time and contexts. The five main behavioural trait dimensions of animal personality are activity, aggressiveness, boldness, exploration and sociability. Animal personalities occur in vertebrates as well as in invertebrates, such as arthropods, yet the ontogenetic development of personality remains elusive. This study investigated the repeatability of activity in immature and mature, and mated and unmated Phytoseiulus persimilis females. To assess behavioural repeatability, P. persimilis individuals were isolated on leaf discs from the early protonymphal stage onwards. Upon reaching adulthood, females were randomly assigned to one of two treatments. Half of the females were allowed to mate by adding a male to the leaf discs for 24 h, while the other half of females remained unmated. The mites’ activity was repeatedly measured by recording their movement on the leaf discs every 10 min during 1 h bouts. Behavioural assays were repeated on three consecutive days each in nymphs and adults, and egg production was recorded for mated females. On average, nymphs moved less than adults and male nymphs moved less than female nymphs. Intraclass correlation coefficients (ICCs) revealed that nymphs were not consistent in their activity levels, whereas maturation increased consistency. Unmated females were highly significantly repeatable, indicating stable individual differences over time. In contrast, the repeatability of mated females decreased following mating. Additionally, generalized estimating equations (GEE) showed significant differences in mean activity across assays in mated but not unmated females. Together, these results suggest that maturation and mating can alter mean activity and personality expression in P. persimilis females. While unmated females expressed activity as a stable personality trait, mating decreased among-individual variability, likely driven by the relaxed need to find a mate and the physiological demands of egg production. Our findings highlight the importance of ontogenetic development and reproductive state for personality expression in arthropods.