Creating a magnetized relativistic pair plasma in the laboratory would enable the exploration of unique plasma physics relevant to some of the most energetic events in the universe. As a step toward a laboratory pair plasma, we have demonstrated an effective confinement of multi-[Formula: see text] electrons inside a pulsed-power-driven 13 [Formula: see text] magnetic mirror field with a mirror ratio of 2.6. The confinement is diagnosed by measuring the axial and radial losses with magnetic spectrometers. The loss spectra are consistent with [Formula: see text] [Formula: see text] electrons confined in the mirror for [Formula: see text] [Formula: see text]. With a source of 10¹² electron-positron pairs at comparable energies, this magnetic mirror would confine a relativistic pair plasma with Lorentz factor [Formula: see text] and magnetization [Formula: see text].