This paper presents the first MRAM-based In-Memory-Computing (IMC) macro, implemented as a 128-kb array in an advanced-node 22nm FD-SOI technology. The design maximizes IMC row parallelism for energy efficiency and throughput, while addressing the critical challenges this raises, namely: high column currents; high output dynamic-range requirements; and large area of peripheral readout circuits. These are addressed through current-insensitive column-multiplexing and high-sensitivity readout circuits, occupying 26% of the macro area. Residual IMC non-idealities, arising from statistical circuit variations, are modeled and incorporated in a chip-generalized one-time neural-network training algorithm, with CIFAR-10 image-classification accuracy demonstrated at 90.1%, equal to ideal digital computation. The design addresses the particularly high sensitivity required for MRAM-based IMC compared to other non-volatile memory technologies, while achieving area-normalized throughput of 758 GOPS/mm2 and energy efficiency of 5.1 TOPS/w for the macro.