Background Increased physical activity (PA) may mitigate the negative cardiovascular health effects of sedentary behavior in adolescents. However, the relationship of PA and sedentary time from childhood with cardiac function in adolescence remains underexplored. Therefore, we investigated the associations of cumulative sedentary time and PA from childhood to adolescence with cardiac function in adolescence. Methods and Results Participants were 153 adolescents (69 girls) who were aged 6 to 8 years at baseline, 8 to 10 years at 2‐year follow‐up, and 15 to 17 years at 8‐year follow‐up. Cumulative sedentary time and PA exposure between baseline and 2‐year follow‐up and between baseline and 8‐year follow‐up were measured using a combined accelerometer and heart rate monitor. Cardiac function was assessed using impedance cardiography at 8‐year follow‐up. The data were analyzed using linear regression analyses adjusted for age and sex. Cumulative moderate to vigorous PA (standardized regression coefficient [β]=−0.323 [95% CI, −0.527 to −0.119]) and vigorous PA (β=−0.295 [95% CI, −0.508 to −0.083]) from baseline to 8‐year follow‐up were inversely associated with cardiac work at 8‐year follow‐up. Conversely, cumulative sedentary time had a positive association (β=0.245 [95% CI, 0.092−0.398]). Cumulative vigorous PA from baseline to 8‐year follow‐up was inversely associated with cardiac work index at 8‐year follow‐up (β=−0.218 [95% CI, −0.436 to 0.000]). Conclusions Higher levels of sedentary time and lower levels of PA during childhood were associated with higher cardiac work in adolescence, highlighting the importance of increasing PA and reducing sedentary time from childhood.