This study measured physical activity (PA) and explored its correlates among children with multimorbidity (co-occurring chronic physical and mental illness; MM) versus those with chronic physical illness only (PI). This study used baseline data from the Multimorbidity in Children and Youth Across the Life Course (MY LIFE) study, an on-going cohort study following 263 children with a PI 2 to 16 years of age (mean age: 9.8 years, SD = 4.0; 47.7% female). PA was measured using accelerometry, and demographic and psychosocial variables were collected using questionnaires. Of the 55 children with MM and the 85 with PI with valid accelerometer data, 38.1% and 41.2%, respectively, met average daily PA guidelines. Correlates of moderate-to-physical PA (MVPA) among children with MM were age, [rho](53) = -0.45, p = 0.001, body mass index (BMI), [rho](48) = -0.28, p = 0.04, self-perceived behavioral conduct, [rho](24) = -0.45, p = 0.02, physical health-related quality of life, [rho](51) = 0.56, p < 0.001, and peer support, [rho](52) = 0.27, p = 0.04. Correlates of MVPA among children with PI were age, [rho](83) = -0.40, p < 0.001, sex, [rho](83) = -0.26, p = 0.01, self-perceived social competence, [rho](31) = 0.42, p = 0.02, self-perceived athletic competence, [rho](31) = 0.48, p = 0.005, physical health-related quality of life, [rho](83) = 0.34, p = 0.001, participation in community sport, [rho](31) = 0.41, p = 0.02, and family functioning, [rho](83) = 0.26, p = 0.02. These results demonstrate that children with PI and MM are insufficiently active and their PA is correlated with demographic and psychosocial factors.