Objective: The objective of this study was to understand adolescent girls' experiences using practical diet and physical activity measurement tools and to explore the food and physical activity settings that influence their lifestyle habits. Design: Mixed methods study using quantitative and qualitative methods. Setting: Large city in the western USA. Participants were recruited from a larger study of girls and families. Methods: A total of 10 girls aged 13-17 years completed a 3-day food diary, a Food Frequency Questionnaire and wore a Fitbit wrist bracelet to measure physical activity for 30 days. Following the data collection period, a structured focus group using a phenomenological approach was held to understand girls' experiences with the collection tools and to understand the internal and external environments in which they make eating and physical activity decisions. Results: Participants completed an average of 8,459 steps/day. Girls had an average Healthy Eating Index score of 37 (out of 100), with diets being particularly high in added sugars and low in nutrients found in fruits and vegetables. Girls perceived the 3-day food diaries and the Fitbit as representing their lives accurately, and had negative perceptions of the Food Frequency Questionnaire. Finally, girls described influences on their lifestyle behaviours, including boredom, summer-break, part-time work, availability and cost. Conclusion: This study demonstrates the efficacy and willingness of girls to utilise food diaries and activity trackers. Results confirm poor dietary intake and physical activity in this population, and describe influences on lifestyle behaviours unique to adolescent girls.