Importance: Cross-sectional measures of cardiovascular health (CVH) have been associated with cardiovascular disease in older age, but little is known about longitudinal trajectories in CVH and their association with subclinical atherosclerosis in middle age. Objectives: To model long-term patterns in CVH starting in childhood and to assess their association with subclinical atherosclerosis in middle age. Design, Setting, and Participants: This cohort study used data from 5 prospective cardiovascular cohort studies from the United States and Finland from 1973 to 2015. A total of 9388 participants aged 8 to 55 years had at least 3 examinations and were eligible for this study. Statistical analysis was performed from December 1, 2015, to June 1, 2019. Exposures: Clinical CVH factors (body mass index, total cholesterol level, blood pressure, and glucose level) were classified as ideal, intermediate, or poor, and were summed as a clinical CVH score. Group-based latent class modeling identified trajectories in this score over time. Main Outcomes and Measures: Carotid intima-media thickness (cIMT) was measured for participants in 3 cohorts, and high cIMT was defined as a value at or above the 90th percentile. The association between CVH trajectory and cIMT was modeled using both linear and logistic regression adjusted for demographics, baseline health behaviors, and baseline (or proximal) CVH score. Results: Among 9388 participants (5146 [55%] female; 6228 [66%] white; baseline mean [SD] age, 17.5 [7.5] years), 5 distinct trajectory groups were identified: High-late decline (1518 participants [16%]), high-moderate decline (2403 [26%]), high-early decline (3066 [32%]), intermediate-late decline (1475 [16%]), and intermediate-early decline (926 [10%]). The high-late decline group had significantly lower adjusted cIMT vs other trajectory groups (high-late decline: 0.64 mm [95% CI, 0.63-0.65 mm] vs intermediate-early decline: 0.72 mm [95% CI, 0.69-0.75 mm] when adjusted for demographics and baseline smoking, diet, and physical activity; P <.01). The intermediate-early declining group had higher odds of high cIMT (odds ratio, 2.4; 95% CI, 1.3-4.5) compared with the high-late decline group, even after adjustment for baseline or proximal CVH score. Conclusions and Relevance: In this study, CVH declined from childhood into adulthood. Promoting and preserving ideal CVH from early life onward may be associated with reduced CVD risk later in life.