Importance Higher resting heart rate (RHR) is associated with increased risk of cardiovascular and all-cause mortality. Limited attention has been paid to early-life determinants of life-course RHR. Objective To describe trajectories of RHR in the same individuals from age 6 to 69 years. Design, Setting, and Participants Data were from the Medical Research Council National Survey of Health and Development population-based cohort of individuals who were followed up from birth in 1946 until 2015. Analysis was conducted between September 2016 and June 2017. Multilevel models were used to estimate life-course mean RHR trajectory by sex and to investigate associations with early childhood factors. The maximal sample included participants who had at least 1 measure of RHR since study entry and a measure of birth weight (N = 4779; observations = 26 182). Exposures Information on early-life factors was ascertained prospectively: childhood socioeconomic position, birth weight, body mass index (calculated as weight in kilograms divided by height in meters squared) change from age 2 to 6 years (conditioned on body mass index at age 2 years), duration of breastfeeding, and markers of neurodevelopment (age at first walking independently and cognitive ability at age 8 years). Main Outcomes and Measures Resting heart rate measured on 8 occasions between age 6 and 69 years (3 occasions in childhood and 5 in adulthood). Results Of 4779 participants, 2492 (52.1%) were women, and 2287 (47.9%) were men. Mean estimated RHR decreased with increasing age and plateaued in adulthood. In sex-adjusted analyses, higher birth weight and conditional body mass index change were associated with lower RHR at age 6 years and across the life course (–0.56 bpm [95% CI, –0.95 to –0.17] per 1 kg higher birth weight and –0.30 bpm [95% CI, –0.48 to –0.13] per 1 kg/m2 change in body mass index). Associations between socioeconomic position and breastfeeding on RHR trajectory emerged in adulthood such that by age 69 years, RHR was 1.48 bpm (95% CI, 0.45 to 2.51) higher in participants from a disadvantaged vs advantaged background and –1.34 bpm (95% CI, –2.39 to –0.29) lower for those who were breastfed for 8 months or longer vs never. A later age at first walking was associated with higher RHR at age 6 years (1.49 bpm [95% CI, 0.39 to 2.59] higher for those 18 months or older vs those younger than 12 months) but with lower RHR in adulthood (–1.18 bpm [95% CI, –2.75 to 0.39] at age 69 years). Conclusions and Relevance Early life is a key period in determining future RHR trajectories with associations with potentially modifiable factors persisting into the seventh decade of life.