IMPORTANCE: Understanding the long-term impact of long COVID on physical function and health-related quality of life (HRQoL) is essential to guide clinical care and rehabilitation strategies. OBJECTIVE: The objective of this study was to compare physical capacity over time among adults without COVID-19 (control group), those who recovered from COVID without persistent symptoms (short COVID group), and those with long COVID (long COVID group [LCG]). A secondary objective was to identify baseline factors predicting HRQoL 6 months later in the LCG. DESIGN: This study was a prospective longitudinal cohort study. SETTING: Assessments were conducted at baseline, 3 months, and 6 months as part of in-laboratory evaluations performed either at the Center for Interdisciplinary Research in Rehabilitation and Social Integration (CIRRIS) in Quebec City or at the Orthopedic Clinical Research Unit of the Maisonneuve-Rosemont Hospital Research Center in Montreal. PARTICIPANTS: A total of 360 age- and sex-matched adults (n = 120 per group), including individuals without a history of COVID-19 (CG), those with short COVID (symptom resolution within 4 weeks, SCG), and those with long COVID (symptoms persisting ≥12 weeks, LCG) participated in the study. INTERVENTION/EXPOSURE: Participants were categorized based on their COVID-19 history and symptom duration and no intervention or exposure was applied. MAIN OUTCOMES AND MEASURES: Self-reported outcomes measuring HRQoL, comorbidities, sleep quality, pain, and fatigue, along with objective performance measures such as grip strength, Short Physical Performance Battery, 6-minute walk test (6MWT), and perceived exertion (Modified Borg Scale) during the 6MWT, were collected at each time point. Daily averages for resting heart rate, step count, and minutes of intensive activity were recorded over 7 days using a fitness tracker watch. Generalized estimating equations were used for longitudinal comparisons, and recursive partitioning analysis for predicting HRQoL factors. RESULTS: Significant time × group interactions were observed for HRQoL, sleep quality, pain, fatigue, Short Physical Performance Battery, and 6MWT. Although the LCG showed significant improvements across these outcomes, only the reduction in fatigue reached a clinically meaningful level, whereas the other groups remained stable. A group effect was detected for all outcomes, except for heart rate and minutes of intensive activity, with the LCG consistently showing lower scores across all follow-ups. Recursive partitioning analysis identified 2 baseline predictors of HRQoL at 6 months in the LCG: self-reported fatigue and daily step count. CONCLUSIONS AND RELEVANCE: These findings highlight the persistent impairments in adults with long COVID and emphasize early HRQoL predictor identification to anticipate long-term needs and adjust treatment plans accordingly.