In the last decade there has been a rise in the availability of consumer focused physical activity and fitness tracking devices. Recently there has been interest in using these devices from the research community for data collection and as part of health behavior interventions. With millions of adults using activity trackers it is necessary to develop an understanding of how they are used, and to what extent different factors may affect physical activity outcomes. The current study sought to explore the relationship between behavior change techniques (BCTs) used by long-term users of Fitbit activity tracking devices and change in physical activity over time. In-depth interviews were conducted with participants in order to obtain information about behavior change techniques connected to the use of the Fitbit system. Historical Fitbit data (steps and activity intensity) were also collected. Thirty participants were recruited to take part in the study. Based on coding of the in-depth interviews, individuals who are long-term users of physical activity tracking devices were found to use a variety of techniques associated with their engagement with the devices and its connected applications. On average participants took 9,695 steps (SD = 5,309) and participated in 33.90 minutes of MVPA (SD = 42.90) per day. An exploration of the relationship between technique use and physical activity outcomes using multi-level modeling indicated that there was limited support for the relationship between use of techniques included in the design the Fitbit system and positive change in physical activity over time. No support was found for a positive relationship between the use of additional BCTs not included in the design of the Fitbit system and physical activity outcomes. Additional qualitative analysis demonstrated that individuals think about and use the same techniques in different ways and apply them in different contexts. This exploratory study is the first examination of BCT use by individuals who freely choose to use physical activity tracking devices, and provided a proof of concept for a systematic mixed methods approach. Results of the study highlight the importance of understanding context of behavior change technique use in relation to physical activity behavior change.