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Mobile operators are poised to leverage millimeter wave technology as 5G evolves, but despite efforts to bolster their reliability indoors and outdoors, mmWave links remain vulnerable to blockage by walls, people, and obstacles. Further, there is significant interest in bringing outdoor mmWave coverage indoors, which for similar reasons remains challenging today. This paper presents the design, hardware implementation, and experimental evaluation of mmWall, the first electronically almost-360 degree steerable metamaterial surface that operates above 24 GHz and both refracts or reflects incoming mmWave transmissions. Our metamaterial design consists of arrays of varactor-split ring resonator unit cells, miniaturized for mmWave. Custom control circuitry drives each resonator, overcoming coupling challenges that arise at scale. Leveraging beam steering algorithms, we integrate mmWall into the link layer discovery protocols of common mmWave networks. We have fabricated a 10 cm by 20 cm mmWall prototype consisting of a 28 by 76 unit cell array, and evaluate in indoor, outdoor-to-indoor, and multi-beam scenarios. Indoors, mmWall guarantees 91% of locations outage-free under 128-QAM mmWave data rates and boosts SNR by up to 15 dB. Outdoors, mmWall reduces the probability of complete link failure by a ratio of up to 40% under 0-80% path blockage and boosts SNR by up to 30 dB.