The human ClpXP complex (hClpXP) plays a central role in mitochondrial protein quality control by degrading misfolded or unneeded proteins. While bacterial ClpXP complexes have been extensively characterized, the molecular determinants underlying hClpXP assembly and regulation are not as well understood. We determined cryo-electron microscopy (cryo-EM) structures of hClpP in isolation and in complex with hClpX, revealing how hClpX binding promotes rearrangement of an asymmetric hClpP heptamer to assemble as a symmetric tetradecamer. Our hClpXP structure also highlights the stabilizing role of a previously uncharacterized eukaryotic ClpX sequence, referred to as the E-loop, and its importance in ATPase activity and hexamer assembly. We further show that peptide interaction with the hClpP proteolytic active site promotes the complex to adopt a proteolytically competent conformation. Together, these findings advance our understanding of the molecular mechanisms defining hClpXP activation and function.