Background. The effectiveness of community-based geriatric intervention models for vulnerable older adults is controversial. We evaluated a problem-based multidisciplinary intervention targeting vulnerable older adults at home that promised efficacy through better timing and increased commitment of patients and primary care physicians. This study compared the effects of this new model to usual care. Methods. Primary care physicians referred older people for problems with cognition, nutrition, behavior, mood, or mobility. One hundred fifty-one participants (mean age 82.2 years, 74.8% women) were included in a pseudocluster randomized trial with 6-month follow-up for the primary outcomes. Eighty-five participants received the new intervention, and 66 usual care. In the intervention arm, geriatric nurses visited patients at home for geriatric assessment and management in cooperation with primary care physicians and geriatricians. Modified intention-to-treat analyses focused on differences between treatment arms in functional abilities (Groningen Activity Restriction Scale-3) and mental well-being (subscale mental health Medical Outcomes Study [MOS]-20), using a mixed linear model. Results. After 3 months, treatment arms showed significant differences in favor of the new intervention. Functional abilities improved 2.2 points (95% confidence interval [CI], 0.3–4.2) and well-being 5.8 points (95% CI, 0.1–11.4). After 6 months, the favorable effect increased for well-being (9.1; 95% CI, 2.4–15.9), but the effect on functional abilities was no longer significant (1.6; 95% CI, −0.7 to 3.9). Conclusions. This problem-based geriatric intervention improved functional abilities and mental well-being of vulnerable older people. Problem-based interventions can increase the effectiveness of primary care for this population.