1. Given their vulnerability to local extinction, the reintroduction of megafauna species (often long-lived, ecologically-influential and highly-social) is an increasingly relevant conservation intervention. Studies that evaluate past megafauna reintroductions are both critical and rare. 2. Between 1981 and 1996, 12 cohorts of a total of 200 juvenile (<5 years old) African savanna elephants (Loxodonta africana africana) were re-introduced to Hluhluwe-iMfolozi Park (HiP), South Africa, after 100 years of absence. Here we model the population’s long term growth. We also present data on the current (2016) age class distribution and social dynamics of the population based on a year of intensive vehicle-based monitoring of 16 collared adult females and their family groups. 3. Exponential population growth (7.1% annual increase) between 1996 and 2014 suggests reintroduction success but has created concerns about overpopulation (with contraception implemented since 2014 to suppress reproduction). The age class distribution has normalised as the juveniles have aged; reproductive females (>10 years old) composed 30% of the population in 2016. The population remains relatively young and forecasts suggest high potential for sustained growth over the next decade. 4. The first calf was born to a reintroduced female in 1990 and since then mother-calf units have gradually developed into semi-independent multi-generation families (7-15 individuals in size in 2016). The size of observed cow-calf groups was highly variable (mean=21.4 individuals, range: 7-109), with repeat observation of individual collared females revealing fusion and fission among different family groups through time, as is typical of more natural elephant populations. 5. Synthesis and applications: The development of normal elephant demography and sociality from an irregular founder population may be an encouragement for the reintroduction of other megaherbivores. The potential for rapid population growth must however be carefully considered, especially when ecologically-influential species are introduced to closed systems. The observed age class distribution and the estimated potential for future growth over the next decade have implications for the park’s contraception strategy. Finally, our study provides key long-term insights for elephant translocations, which are becoming an increasingly common and necessary management intervention (due to overpopulation in some areas and local extinction in others).04-Jun-2018