Adolescence is a challenging period, particularly for individuals with autism spectrum disorder (ASD), due to having little knowledge about themselves. Self-management is a strategy to enhance self-understanding through continuous self-monitoring, which can support adaptive transitions to adulthood. Meanwhile, the advent of digital self-tracking tools enables users to collect and reflect on data about themselves. In this work, we investigated how adolescents with ASD kept track of their everyday lives to better understand themselves using a custom self-tracking platform, OmniTrack, over a two-week period. Our findings indicate that personalized self-tracking experiences enable adolescents to monitor the detailed contexts, causes, and consequences of problematic situations; regulate negative emotion and anxiety while interacting with the tracker; and communicate through data with their caregivers, teachers, and therapists. Building on these findings, we suggest the design of a new form of flexible, scaffolded self-tracking technique that can inform both researchers for designing pervasive health technologies for adolescents with ASD and clinicians for guiding adolescents with ASD toward better self-management using such technologies.