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Long-acting lenacapavir (LEN) has emerged as a highly effective, potentially game-changing therapy for HIV treatment and prevention ^1–5 . Although its mechanism of action in the early phase of HIV-1 replication, when the capsid directs key post-entry steps such as reverse transcription, nuclear import, and integration, has been well characterized ^1,2,6–8 , its effects during the late phase of replication, when the capsid assembles and matures within budding virions, remain poorly understood. Here, we determine the cryo-electron microscopy structure of the mature HIV-1 capsid lattice assembled within virus-like particles in the presence of LEN. Our structural analyses revealed that LEN alters interhexamer interactions, perturbs capsid lattice curvature, and thereby prevents the formation of a functional cone-shaped capsid. Biochemical analyses further demonstrated that LEN-containing cores lose reverse transcriptase due to a compromised capsid integrity, whereas integrase and viral RNA remain associated. Functionally, viruses produced in the presence of LEN exhibited markedly reduced infectivity, low reverse transcription activity, and poor integration. Together, these findings provide mechanistic insight into the late-phase action of LEN and provide key directions for the design of future inhibitors.