Deformation tests at room temperature have been carried out on a series of in situ Mg-Mg17Al12 composites with volume fraction of Mg17Al12 phase varying between 20 % and 60%. Regardless of the phase volume fraction, these Mg-Mg17Al12 composites that are prepared by solidification consist in magnesium-rich dendrites and a eutectic mixture. The eutectic mixture is made of magnesium fibres embedded in a matrix formed by the Mg17Al12 phase, a hard and brittle complex metallic alloy. Compression tests at room temperature have shown that a global plastic deformation is observed even though the soft phase is always enclosed in the brittle one. According to the strain-stress curve, deformation proceeds in two stages: a first stage at low stress (~ 100 MPa) and low strain ( < 0.01) almost similar for all the composites and a second stage characterized by a strain hardening effect along with a significant deformation for composites with  phase volume fraction smaller than 40 %, whereas no or little additional deformation is observed for composites with volume fraction higher than 40% that break at high stress (~ 400 MPa). Microstructural observations of the deformed samples have evidenced dislocations in the Mg17Al12 matrix and their localisation as pileups and slip bands as well as the transmission of deformation across the eutectic mixture. The plasticity of the Mg17Al12 phase and the mechanical behaviour of the Mg-Mg17Al12 composites are discussed with respect to the microstructure characteristics and to the specific atomic features of complex metallic alloy phases.