Despite high risk for HIV, South African adolescent girls and young women (AGYW) experience numerous challenges with adherence to PrEP. To better understand AGYW's motivations for PrEP and factors that impact PrEP adherence, we conducted serial in-depth interviews with 22 South African AGYW during a 12 month prospective study. Interviews explored motivations and initial experiences of PrEP use, patterns of adherence, social support, and reasons for stopping or persisting with PrEP. We also assessed drug levels as a biomarker of adherence; dried blood spots were collected at months 1, 2, 3, 6, and 12 to assess intracellular tenofovir diphosphate levels. An end-user journey analytical approach revealed themes related to behavioral and emotional aspects of use, including multilevel factors leading to divergent PrEP adherence trajectories. Our findings highlight how internal versus external motivations drive PrEP use, as well as how positive identity formation and challenges are handled, which are essential to understand AGYW in their PrEP journeys.