Ancient duplication events and retained gene duplicates have contributed to the evolution of many novel plant traits and, consequently, to the diversity and complexity within and across plant lineages. While mounting evidence highlights the importance whole-genome duplication (WGD; polyploidy) and its key role as an evolutionary driver, gene duplication dynamics and mechanisms, both of which are fundamental to our understanding of evolutionary process and patterns of plant diversity, remain poorly characterized in many clades. We use newly available transcriptomic data and a robust phylogeny to investigate the prevalence, occurrence, and timing of gene duplications in Lamiaceae (mints), a species-rich and chemically diverse clade with many ecologically, economically, and culturally important species. We also infer putative WGDs—an extreme mechanism of gene duplication—using large-scale data sets from synonymous divergence (K_S), phylotranscriptomic, and divergence time analyses. We find evidence for widespread but asymmetrical levels of gene duplication and ancient polyploidy in Lamiaceae that correlate with species richness, including pronounced levels of gene duplication and putative ancient WGDs (7–18 events) within the large subclade Nepetoideae and up to 10 additional WGD events in other subclades. Our results help disentangle WGD-derived gene duplicates from those produced by other mechanisms and illustrate the non-uniformity of duplication dynamics in mints, setting the stage for future investigations that explore their impacts on trait diversity and species diversification. Our results also provide practical context for evaluating the benefits and limitations of transcriptome-based approaches to inferring WGD, and we offer recommendations for researchers interested in investigating ancient WGDs in other plant groups.