Investigations into the effects of magnesium content and cooling rates on the mechanical properties of aluminium-magnesium alloys refined with titanium-boride master alloy was conducted. Experimental samples were produced from melt with Mg content ranging from 0% to 8%, poured and cooled in air, water and oil. Standard sample dimensions were produced for tensile and hardness tests. It was observed that both the Mg content and the cooling rate affect the mechanical properties; elongation (ductility) reduced with increased Mg content while the hardness of the samples increased with the Mg content. It was concluded that the sample poured and cooled in water possessed the optimum mechanical properties (combination of ductility and hardness) within the range of composition of maximum ultimate tensile strength (UTS).