Red milk mushroom (Lactarius hatsudake) increased antioxidant activity and beneficial compounds in yogurt, with higher concentrations (up to 3.0% addition) driving the strongest gains in phenolics and free-radical scavenging capacity. It also improved microbial stability, increasing live beneficial bacteria counts (LAB) over storage while maintaining quality across 28 days of refrigeration, indicating better shelf life and fermentation performance. Lower doses (~0.5%) preserved texture and taste while still boosting function, suggesting a practical formulation that enhances everyday yogurt without compromising quality.