Improving the capacity for physical activity interventions to maintain behavior change is a key public health concern and an important strategy for the health and independence of older adults. Ways of ensuring effective maintenance of physical activity levels in older adults are unclear. This study includes the objective measure of moderate-to-vigorous physical activity (MVPA); self-reported self-efficacy; and self-regulation at four timepoints (baseline, intervention completion at 4 months, 12-, and 18-month follow-up) from the SITLESS study, a clinical trial conducted with a cohort of community-dwelling older adults (≥65 years) from Denmark, Germany, Spain, and the United Kingdom. A cross-lagged analysis found that self-regulation and self-efficacy may be key determinants of MVPA behavior in community-dwelling older adults. More specifically, the use of behavioral support strategies represents an important correlate of MVPA behavior, and its association with MVPA may be mediated by self-regulation and self-efficacy in older adults in the short and long term.