The range contractions and expansions due to climate fluctuations during the Quaternary have profoundly shaped the evolutionary history of species and populations. In this thesis, I used ancient, historical, and modern mitochondrial and whole genomes to study the postglacial population history of Scandinavian mammals in the context of past climate changes and human actions. In Chapter I, I analysed 41 ancient, historical, and modern mitogenomes of Scandinavian brown bears (Ursus arctos), to gain further insights into the Holocene history of the species. I found that southern Scandinavia was likely recolonised by several female lineages, whereas northern Scandinavia was recolonised by only a single lineage after the last glaciation. Moreover, a recent population bottleneck resulted in a severe loss of mitochondrial genetic diversity, with only a single haplotype remaining in southern Scandinavia today. In Chapter II, I used seven historical and 21 modern high-coverage nuclear genomes of Scandinavian brown bears to study the genome-wide effect of the population decline ca. 100 years ago. The results suggested that while the individual genomic diversity and inbreeding levels were maintained, the population structure was reshaped as a consequence of the bottleneck, where our analyses suggest that male-biased gene flow resulted in a mitochondrial-nuclear discordance. Additionally, the overall population diversity likely declined as the unique genetic variation of southern Scandinavia mostly was lost. In Chapter III, I generated 86 modern Norwegian lemming (Lemmus lemmus) genomes, and investigated the present-day population structure of this small rodent endemic to Fennoscandia. Here I found geographically structured subpopulations which are characterised by an isolation-by-distance pattern, with the highest genetic diversity in lemmings in the northern part of Sweden. Finally, in Chapter IV, I studied the Late Quaternary population dynamics and biogeographic history of Lemmus in western Eurasia, using ancient and modern high-coverage genomes. The results suggest a complex population history of Lemmus sp. over the last 50,000 years, involving hybridisation among divergent lineages and genomic patterns that open new questions about the postglacial recolonisation of Fennoscandia.