Engineering intramolecular exchange interactions between magnetic metal atoms is a ubiquitous strategy for designing molecular magnets. For lanthanides, the localized nature of 4f electrons usually results in weak exchange coupling. Mediating magnetic interactions between lanthanide ions via radical bridges is a fruitful strategy towards stronger coupling. In this work we explore the limiting case when the role of a radical bridge is played by a single unpaired electron. We synthesize an array of air-stable Ln ₂ @C ₈₀ (CH ₂ Ph) dimetallofullerenes (Ln ₂ = Y ₂ , Gd ₂ , Tb ₂ , Dy ₂ , Ho ₂ , Er ₂ , TbY, TbGd) featuring a covalent lanthanide-lanthanide bond. The lanthanide spins are glued together by very strong exchange interactions between 4f moments and a single electron residing on the metal–metal bonding orbital. Tb ₂ @C ₈₀ (CH ₂ Ph) shows a gigantic coercivity of 8.2 Tesla at 5 K and a high 100-s blocking temperature of magnetization of 25.2 K. The Ln-Ln bonding orbital in Ln ₂ @C ₈₀ (CH ₂ Ph) is redox active, enabling electrochemical tuning of the magnetism.