Improving fluid intelligence is an enduring research aim in the psychological and brain sciences that has motivated public interest and scientific scrutiny. At issue is the efficacy of prominent interventions—including fitness training, computer-based cognitive training, and mindfulness meditation—to improve performance on untrained tests of intellectual ability. To investigate this issue, we conducted a comprehensive 4-month randomized controlled trial in which 424 healthy adults (age 18–43 years) were enrolled in one of four conditions: (1) Fitness training; (2) Fitness training and computer-based cognitive training; (3) Fitness, cognitive training, and mindfulness meditation; or (4) Active control. Intervention effects were evaluated within a structural equation modeling framework that included repeated-testing gains, as well as novel tests of fluid intelligence that were administered only at post-intervention. The combination of fitness and cognitive training produced gains in visuospatial reasoning that were greater than in the Active Control, but not in performance on novel tests administered only at post-intervention. Individuals more variably responded to multi-modal training that additionally incorporated mindfulness meditation (and less time spent on cognitive training), and those who demonstrated repeated-testing gains in visuospatial reasoning also performed better on novel tests of fluid intelligence at post-intervention. In contrast to the multi-modal interventions, fitness only training did not produce Active Control-adjusted gains in task performance. Because fluid intelligence test scores predict real-world outcomes across the lifespan, boosting intelligence ability via multi-modal intervention that is effective even in young, healthy adults is a promising avenue to improve reasoning and decision making in daily life.