Streptococcus pyogenes, or Group A Streptococcus (GAS), is a strictly human bacterial pathogen with a global distribution that is often associated with mild infections of the respiratory tract (pharyngitis) and the skin (impetigo). If left untreated pharyngitis can lead to further complications, including scarlet fever which caused devastating epidemics through the 19th and early 20th centuries. After decades of decline, a sharp increase in scarlet fever cases has again been reported in recent years in northeast Asia and the United Kingdom. Recently, we have shown that GAS emm12 isolates represent the majority of scarlet fever cases since the outbreak in mainland China and Hong Kong in 2011. Genomic epidemiological analyses indicated that acquisition of mobile genetic elements including the prophage ΦHKU.vir encoding superantigens SSA, SpeC and the DNase Spd1 triggered the expansion of scarlet fever–associated emm12 lineages in Hong Kong. To further explore how these exotoxins contribute to the fitness and evolution of GAS, we generated isogenic single and triple knockout mutant strains of ssa, speC and spd1 of the published Hong Kong scarlet fever strain HKU16. We show that loss of all three exotoxins together (SSA, SpeC and Sda1) significantly reduced nasopharyngeal colonization in a mouse model of streptococcal infection, whereas neither single mutant showed any substantial reduction in bacterial colonization. Our finding suggests a synergistic relationship between these exotoxins and that acquisition of ΦHKU.vir played a key role in selection and expansion of scarlet fever lineages throughout China.