Inbreeding, the mating between related individuals, often adversely affects individual fitness in a phenomenon known as inbreeding depression (ID). Small and genetically-isolated populations are especially vulnerable to ID, making understanding its causes and consequences important from a wildlife conservation perspective. Quantifying ID in wildlife is challenging, as there are difficulties with collecting long-term life history data needed to estimate fitness and high-quality genetic data needed to precisely estimate inbreeding, especially in free-living populations. In my thesis, I studied ID in the isolated feral horse population of Sable Island, Nova Scotia, by combining individual-based genomic, ecological and life history data from 239 individuals. Genomic data was generated using Single Nucleotide Polymorphism (SNP) arrays, and inbreeding levels were quantified through genomic estimators based on SNPs found in runs of homozygosity, FROH, correlation between uniting gametes, FUNI, and pairwise relatedness. I tested for presence of ID on sex-specific annual adult reproductive success and survival as well as foal births, and the influence of intrinsic and extrinsic variables in modulating ID using generalized linear mixed models and model averaging. Low overall levels of inbreeding was found in the population, and inbreeding was not a significant predictor of any of the fitness traits studied when considered on its own. However, results suggested that ID for female annual reproductive success, female annual survival, and male annual survival may be modulated by environmental conditions such as band size, location, and winter severity. The performance of the estimators FROH and FUNI were similar throughout my analyses. Using a standardized measure of ID, I also show that ID in Sable Island horses is relatively low in comparison to other wildlife populations. This research expands our knowledge of inbreeding depression in natural systems and provides Parks Canada with important information to assist with the management and conservation of Sable Island’s iconic population of feral horses.