In 2016/2017, Washington State experienced a mumps outbreak despite high childhood vaccination rates, with cases more frequently detected among school-aged children and members of the Marshallese community. Sequencing 166 mumps genomes revealed that mumps was introduced into Washington approximately 13 times, primarily from Arkansas, sparking multiple, co-circulating transmission chains. Neither vaccination status nor age were strong determinants of transmission. Instead, the outbreak in Washington was overwhelmingly sustained by transmission within the Marshallese community. Our findings underscore the utility of genomic data to clarify epidemiologic factors driving transmission, and pinpoint contact networks as critical determinants of mumps transmission in Washington.