Subjective cognitive decline (SCD) is increasingly regarded as a preclinical stage of dementia with elevated risk for later cognitive impairment. Multidomain lifestyle interventions are promising for dementia prevention, but few have been tailored to community-dwelling older adults with SCD in Korea. This study examined the effects of a lifestyle-based intervention on cognitive function, instrumental activities of daily living (IADL), quality of life, and self-efficacy. Twenty older adults (≥65 years) with SCD were recruited from a dementia safety center and randomly assigned to an experimental group (n = 10) or a control group (n = 10). The experimental group completed an 8-week lifestyle-based program adapted from the My Healthy Brain model, integrating physical activity, nutrition, cognitive stimulation, social engagement, sleep hygiene, stress management. Outcomes (MoCA-K, K-IADL, WHOQOL-BREF, GSES) were analyzed using Wilcoxon signed-ranks and Mann–Whitney U tests. The experimental group showed significant pre–post improvements in all outcomes (p < .05), whereas the control group showed no significant changes. These findings suggest that a multidomain lifestyle-based intervention may enhance cognition, daily functioning, quality of life, and self-efficacy in older adults with SCD and support the feasibility of Korean community-based early intervention models.