Long-period radio transients are a new class of astrophysical objects that exhibit periodic radio emission on timescales of tens of minutes. Their true nature remains unknown; possibilities include magnetic white dwarfs, binary systems, or long-period magnetars; the latter class are predicted to produce fast radio bursts (FRBs). Using the MeerKAT radio telescope, we conducted follow-up observations of the long-period radio transient GPM J1839–10. Here we report the source exhibits a wide range of unusual emission properties, including polarization characteristics indicative of magnetospheric origin, linear-to-circular polarization conversion, and drifting sub-structures closely resembling those observed in repeating FRBs. These radio characteristics provide clear evidence in support of the long-period magnetar model and establish the missing link between long-period radio transients, magnetars, and FRBs.