Digital citizen science platforms are prominent examples of modern volunteerism that provide people with opportunities to observe natural phenomena and to engage in scientific processes. In this study, we explore the values and motivations underlying sustained participation in digital citizen science projects through the lenses of two social psychology theories (Schwartz’s Human Values Theory and Self-Determination Theory). We present in-depth analyses of interviews with 15 long-term volunteers in two digital citizen science initiatives (Järviwiki and Safecast) that have been collecting environmental data for a decade in Finland and Japan. Our results advance the understanding of the values underpinning motivations. Our analyses show that openness-to-change values, such as self-direction, are important for initial participation, yet a diverse range of values, except for power, play a role in sustaining participation. Our study also shows that the values related to sustained participation are linked with extrinsic motivations, suggesting that when extrinsic motivators are self-directed, people will not only perform tasks willingly and enthusiastically but also in a sustained manner. Conceptualizing the behavioral continuum that drives volunteering actions provides practical insights that can assist the design, development, and evaluation of digital citizen science platforms.