Abstract Phosphorus‐rich 6‐MnP 4 nanoparticles are synthesized via high energy mechanical milling (HEMM) and their electrochemical properties as an anode for lithium‐ion batteries (LIBs) and sodium‐ion batteries (SIBs) are investigated focusing on the electrochemical activity and reaction mechanism. The 6‐MnP 4 nanoparticles with a triclinic structure ( P‐1 ) are successfully synthesized by HEMM and they are composed of 5 to 20 nm‐sized crystallites. During the lithiation process, the MnP 4 phase undertakes the sequential alloying (MnP 4 + 7 Li + + 7 e − → Li 7 MnP 4 ) and conversion (Li 7 MnP 4 + 5 Li + + 5 e − → Mn 0 + 4 Li 3 P) reactions. On the other hand, the MnP 4 nanoparticles are directly converted to Mn 0 and Na 3 P without the formation of an intermediate Na–Mn–P alloy phase during sodiation process. The MnP 4 electrode shows high initial discharge and charge capacity (1876 and 1615 mAh g −1 for LIBs, and 1234 and 1028 mAh g −1 for SIBs) and high initial Coulombic efficiency (86% for LIBs and 83% for SIBs), indicating a promising candidate for high capacity anodes. In addition, the long‐term cyclability and high rate capability of MnP 4 can be further improved through the formation of MnP 4 /graphene nanocomposites and vanadium substituted Mn 0.75 V 0.25 P 4 solid solutions.