The crystal and magnetic structures of the two related phases, Sr2MnGaO5 and Ca2MnAlO5 are reported. Rietveld analysis of neutron powder diffraction has revealed that both phases adopt the brownmillerite structure. Subtle differences in structure lead to the structure of Sr2MnGaO5 being best described by the space group Icmm (a = 5.4888(2) Å, b = 16.2256(6) Å, c = 5.35450(2) Å at 2 K) while that of Ca2MnAlO5 is best described by Ibm2 (a = 5.46258(9) Å, b = 14.9532(3) Å, c = 5.23135(8) Å at 2 K). Low temperature neutron powder diffraction data show that both phases have a simple antiferromagnetic structure. However, magnetisation data suggest a more complex picture of the magnetic order within these phases.