Casein, an organic milk protein, existed in masonry mortars from medieval times until the 18th century. It was understood to improve workability of the mortar. Recent mortar conservation projects have proposed its use, however, little information is existent on its effects on strength, a critical property in repair of masonry mortars. Adding 0.25% casein by mass was insufficient to improve flow properties of mortar. It was found that adding 0.5% casein by mass resulted in a flowable mortar, however, a 75% reduction in strength resulted, while bond strength increased, after 56 days. Reducing the water by 18% resulted in a 50% reduction in strength from that observed in the control sample, while maintaining flowable properties. Reducing the water more than 27% yielded a rapid-setting, non-flowable mortar. The addition of casein is a plausible alternative for repointing existing mortar joints. It has favourable flow properties, as well as improved bond strength.