Highly pathogenic avian influenza viruses (HPAIVs) of the H5 subtype pose a continuous threat to animal and public health due to their zoonotic potential, rapid evolution, and ability to spread across continents. Since the emergence of the A/goose/Guangdong/1/96 (GsGD) H5 lineage in 1996, several clades have caused devastating outbreaks in poultry and wild bird populations, occasionally resulting in human infections. Of the many clades that have evolved, only three—clades 2.2, 2.3.2.1, and most recently 2.3.4.4b—have demonstrated the ability to spread globally. The 2.3.4.4b clade has raised significant concern due to its continuous geographic expansion, establishment in new ecosystems, including Antarctica, and increasing reports of mammalian infections, including companion animals, marine mammals, and livestock. Recently, cow-to-cow and cow-to-human transmission marked a paradigm shift in the epidemiology of avian influenza and emphasized the need for continued surveillance. This review summarizes the historical emergence, global spread, and molecular evolution of H5 HPAIVs with a specific focus on the recent expansion of clade 2.3.4.4b and its capacity for mammalian spillover.