Abstract Objectives Previous research has shown the capacity for mindfulness to be strongly associated with psychological well-being, that components of mindfulness show significant growth through young adulthood, and that this developing, malleable capacity is vital as individuals learn to deal appropriately with negative thoughts and unwelcome emotions. Smartphones, typically used in an automatic or experientially avoidant way, can undermine this development, leading to a decreased capacity for mindfulness. The purpose of these studies were to examine the extent to which smartphone use is negatively associated with young adults’ mindfulness and the degree to which increased cognitive and behavioral involvement with smartphones may exacerbate this relation using a newly developed conceptual model. Method Study 1 was conducted using self-report measures of mindfulness among a cross-sectional sample of university students aged 18–20 years (n = 668). Study 2 augmented Study 1 using objective measures of smartphone screen time and the cognitive regulatory components of mindfulness in a planned missingness design. Results Results indicate smartphone involvement (a compulsive pattern of use and cognitive preoccupation with one’s smartphone) to be significantly associated with lower trait mindfulness. Additionally, exploratory analysis of smartphone involvement as a mediator of the effect of smartphone use on mindfulness demonstrated a significant estimated indirect effect. Conclusion These results provide preliminary empirical support for the newly proposed conceptual model which posits associations between mindfulness and the use of smartphones in a cognitively and behaviorally involved way. Preregistration This study is not preregistered.