  The signatures of dark matter at the LHC commonly involve, in simplified
scenarios, the production of a single particle plus large missing energy, from
the undetected dark matter. However, in $Z'$-portal scenarios anomaly
cancellation requires the presence of extra dark leptons in the dark sector. We
investigate the signatures of the minimal scenarios of this kind, which involve
cascade decays of the extra $Z'$ boson into the dark leptons, identifying a
four-lepton signal as the most promising one. We estimate the sensitivity to
this signal at the LHC, the high-luminosity LHC upgrade, a possible high-energy
upgrade, as well as a future circular collider. For $Z'$ couplings compatible
with current dijet constraints the multilepton signals can reach the $5\sigma$
level already at Run 2 of the LHC. At future colliders, couplings two orders of
magnitude smaller than the electroweak coupling can be probed with $5\sigma$
sensitivity.
