Officinal European Leonurus cardiaca, East Asian L. japonicus, and South African Leonotis leonurus are traditionally used for cardiovascular, gynecological, and neurological disorders. Nevertheless, a phytochemical assessment as a basis for their quality control and comparison amongst them has not yet been reported up to now. Here, a novel RP-HPLC method is presented for quantifying twelve phenolics, lavandulifolioside, verbascoside, hyperoside, ferulic acid, isoquercitrin, rutoside, apigenin-7-0-D-glucoside, and quercitrin, as well as chlorogenic, caffeic, rosmarinic, and cichoric acids, in 18 herbal and fruit samples of the three species, as well as in a L. cardiaca refined extract. Only ferulic acid was found in every sample, whereas rosmarinic acid and apigenin-7-0-D- glucoside were not detected in any sample. Chlorogenic, caffeic, and cichoric acids and rutoside were detected in all three species. Lavandulifolioside and verbascoside, the dominant phenolics of L. cardiac, were not present in any sample of L. japonicus. Lavandulifolioside was found in this first ever HPLC analysis on phenolics of L. leonurus. Hyperoside was not found in L. cardiaca, but in both L. japonicus and L. leonurus, whereas isoquercitrin was detected in L. cardiaca and L. leonurns, but not in L. japonicus. This approach facilitates identification and quality control via HPLC/HPTLC fingerprints.