ABSTRACT Purpose The McMaster Monitoring My Mobility (MacM3) study aims to understand trajectories of mobility decline in later life using multi-sensor wearable technology. To our knowledge, MacM3 is the first major cohort to combine accelerometry and GPS to track real-world mobility in community-dwelling older adults. Participants Between May 2022 and May 2024, MacM3 recruited 1,555 community-dwelling older adults (mean age 73.9 years, SD = 5.5) from Hamilton and Toronto, Ontario. Of the cohort, 68.4% were female, 62.4% married/partnered, 75.3% had post-secondary education, and 63.0% had ≥3 comorbidities. Most were Canadian born (69.4%) and White/Caucasian (88.0%), with greater ethnocultural diversity observed at the Toronto site. Findings to date At baseline, 56.7% of participants reported no mobility limitations, 15.9% had preclinical limitations, and 27.4% had minor mobility limitations. Mean gait speed for the total sample was 1.23 m/s, with a mean Timed Up and Go time of 9.4 seconds and a 5x sit-to-stand time of 13.0 seconds. A total of 1,301 participants had valid wrist-worn device data, and 1,008 participants who agreed to wear the thigh-worn device had valid data (≥7 days with ≥10 hours of wear per day). Step count data (n = 1008) revealed a mean of 8,437 steps per day (SD = 2,943), with 5,073 steps in the lowest quartile and 12,303 steps in the highest. Future plans Ongoing work aims to develop predictive models of mobility decline by integrating wearable, clinical, and environmental data. Pipeline enhancements will enable GPS/inertial measurement unit fusion to explore mobility-environment interactions and support aging-in-place tools.