Understanding the drivers of successful species invasions is important for conserving native biodiversity and for mitigating the economic impacts of introduced species. However, whole genome resolution investigations of the underlying contributions of neutral and adaptive genetic variation to successful colonization in introduced populations are rare. Increased propagule pressure should result in greater neutral genetic variation, while environmental differences should elicit selective pressures on introduced populations, potentially supporting greater adaptive genetic variation. We investigated neutral and adaptive variation among nine introduced brook trout (Salvelinus fontinalis) populations using whole-genome sequencing (28,490,618 SNPs based on pool-seq). The populations inhabit different, isolated lakes in western Canada and descend from a common source, with an average of ~19 (range of 7-41) generations since introduction. We found no evidence of bottleneck events nor strong evidence of purifying selection, and little support that varying propagule pressure or differences in local environments shaped neutral genetic variation. Putative outlier analysis revealed non-convergent patterns of adaptive differentiation among lakes with minimal outlier loci (0.001%-0.15%) which did not correspond with tested environmental variables, despite conditions that should facilitate stronger adaptive differentiation (e.g. abiotic and biotic environmental differences, propagule pressure differences). Our whole genome sequencing analysis provides an example of an introduction success not strongly influenced by genetic variation and suggests that observed differentiation among introduced salmonid populations can be idiosyncratic, population-specific, or stochastic.