The same-sign tetralepton signature via the mixing of neutral Higgs bosons and their cascade decays to charged Higgs bosons is a unique signal in the type-II seesaw model with the mass spectrum . In this study, we investigate this signature at future lepton colliders, such as the ILC, CLIC, and MuC. Direct searches for doubly charged scalar at the LHC have excluded GeV in the decay mode. Therefore, we choose GeV as our benchmark scenarios. Constrained by direct search, is the only viable decay mode for GeV at the TeV ILC. With an integrated luminosity , the promising region, with approximately 150 signal events, corresponds to a narrow band in the range of GeV. Meanwhile, for GeV at the TeV CLIC, approximately 10 signal events can be produced with . For heavier triplet scalars GeV, although the decay mode is allowed, the cascade decays are suppressed. A maximum event number can be obtained at approximately GeV and for GeV with at the TeV CLIC. Finally, we find that this signature is not promising for GeV at the TeV MuC. Based on the benchmark scenarios, we also study the observability of this signature. In the mode, one can probe GeV at future lepton colliders.