Importance: Progressive loss of muscle mass and strength, known as sarcopenia, is a well-known phenomenon of aging; however, little is known about the trajectory of sarcopenia measures before and after cancer diagnosis and its contribution to subsequent disability. Objective: To examine the rate of decline of sarcopenia measures (ie, appendicular lean mass [ALM], muscle strength, and physical performance) in older adults with cancer both before and after the cancer diagnosis compared with the trajectory of a population without cancer, and secondarily to assess the association of sarcopenia measures with overall survival and major disability in patients with cancer. Design, Setting, and Participants: This matched cohort study included participants from the Health, Aging, and Body Composition (Health ABC) study, which included 3075 community-dwelling older adults aged 70 to 79 years recruited from a random sample of white Medicare beneficiaries and all eligible black residents in and around Pittsburgh, Pennsylvania, and Memphis, Tennessee, beginning in January 1997 and observed for 17 years until December 2013. Data were analyzed from May 2018 to February 2020. Exposures: The development of an adjudicated cancer diagnosis confirmed with pathology or cytology reports during the first 7 years of follow-up. Main Outcomes and Measures: Annual assessments of ALM, hand grip strength, and gait speed were the primary outcome measures. Linear mixed-effect models were used to compare the change in ALM, hand grip strength, and gait speed between individuals who developed cancer and those who did not, adjusted for multiple comparisons (P < .01). Multivariable Cox regression was used to examine the association of sarcopenia measures with overall survival and major disability from date of cancer diagnosis. Results: Of the 3075 included patients, 1491 (48.5%) were male, 1281 (41.7%) were black, and the mean (SD) age was 74.1 (2.9) years. A total of 515 patients (16.7%) developed cancer within the first 7 years of the study. The most common cancers were prostate (117 [23.2%]), colorectal (63 [12.5%]), lung (61 [12.1%]), and breast (61 [12.1%]) cancer, and 165 patients (32.0%) were diagnosed as having metastatic disease. Compared with controls without cancer, patients with a cancer diagnosis had a steeper decline in gait speed (β = -0.02; 95% CI, -0.03 to -0.01; P < .001) but not ALM (β = -0.02; 95% CI, -0.07 to 0.04; P = .49) or hand grip strength (β = -0.21; 95% CI, -0.43 to 0; P = .05) prior to cancer diagnosis. After cancer diagnosis, there was a decline in ALM (β = -0.14; 95% CI, -0.23 to -0.05; P < .001) but not hand grip strength (β = -0.02; 95% CI, -0.37 to 0.33; P = .92) or gait speed (β = 0; 95% CI, -0.01 to 0.02; P = .51). Declines in ALM after a cancer diagnosis were most striking in patients with metastases (β = -0.32; 95% CI, -0.53 to -0.10; P = .003). Slow gait speed was associated with a 44% increase in mortality (hazard ratio, 1.44; 95% CI, 1.05 to 1.98; P = .02) and a 70% increase in disability (hazard ratio, 1.70; 95% CI, 1.08 to 2.68; P = .02) but not low ALM or hand grip strength. Conclusions and Relevance: Accelerated losses in differing sarcopenia measures exist both prior to and after a cancer diagnosis and may present opportunities for targeted interventions to improve outcomes.