Insufficient physical activity (PA) is associated with low cardiorespiratory fitness, which favors cardiovascular and other non-communicable diseases. Besides, it evidentially affects mental health. Considering the WHO PA guidelines, CrossFit® represents a versatile exercise program, combining aerobic and resistance training with mobility and could help lowering disease incidences among sedentary people. Yet, long-term CrossFit research is sparse. We conducted a nine-months intervention (≥ 2 CrossFit workouts/week) in sixteen beginners (14 males, 35  6.8 years, 180  8.6 cm, 85. 5  19.1 kg). As primary endpoint, VO2max was assessed at baseline, after four, and nine months. A repeated-measures ANOVA and pearson correlation were conducted. Well-being was investigated by WHO-5-index pre- and post-intervention. For exploratory purpose, body composition was tracked. Main outcome was an 11.5 % VO2max improvement with large effect (p < .01, η_p^2 = .27). Strong negative correlations between baseline VO2max and its progression after nine months (p = .006, r = -.654) were found. Well-being increased by 8.7 % (p = .024, d = .51). Body composition improved without statistical significance. Resting metabolic rate increased by 2.2 % (p = .042). This study reveals the potential of CrossFit to enhance physiological and psychological health in beginners. For more robust results, bigger sample sizes with higher proportion of women are needed.