We investigated the association of parental physical activity (PA) trajectories with offspring's youth and adult PA. Self-reported PA data were extracted from the Young Finns Study with three follow-ups for parents between 1980 and 1986 and nine follow-ups for their offspring in youth between 1980 and 2011 (aged 9–39 years, n = 2402) and in adulthood in 2018. Accelerometer-derived PA was quantified in 2018–2020 (aged 43–58 years, n = 1134). Data were analyzed using mixture models and conducted in 2022. We identified three trajectories for fathers and mothers (high-stable activity, 20.2%/16.6%; moderate-stable activity, 50.5%/49.6%; and low-stable activity, 29.4%/33.7%) and four for youth male and female offspring (persistently active, 13.4%/5.1%; increasingly active, 32.1%/43.1%; decreasingly active, 14.4%/12.6%; and persistently low-active, 40.1%/39.1%). Compared to low-stable active parents, high-stable active fathers had a higher probability of having their sons and daughters classified as persistently active, increasingly active, and decreasingly active in youth (Brange = 0.50–1.79, all p < 0.008), while high- and moderate-stable active mothers had significantly increased likelihood of having their daughters classified as persistently active and decreasingly active in youth (Brange = 0.63–1.16, all p < 0.009). Fathers' and mothers' high-stable activity was associated with higher self-reported PA of adult offspring than parental low-stable activity. Persistently active and increasingly active offspring in youth accumulated more adult total PA, moderate-to-vigorous PA, step counts, and self-reported PA than persistently low-active ones (all p < 0.036). Parental persistent PA, particularly paternal persistent PA, predicts offspring's PA concurrently and prospectively. Increasing and maintaining PA in youth predicts higher PA levels in midlife.