Pultrusion is an industrial process for the manufacture of fibre-reinforced composites having highly aligned continuous fibre reinforcement. In this work, the effect of processing conditions (temperature) and material composition (filler type, filler content, release agents) on the mechanical properties of pultruded glass fibre-reinforced polymer matrix composites was investigated. The processing-property relationships of the pultruded composites were assessed by measuring the tensile and flexural behaviour, short beam strength and curing behaviour. An empirical model is developed to predict the density of the composites, based on the volume fraction of glass fibre, filler additions and shrinkage behaviour. The study shows that significant improvements in the tensile (>75%), flexural (>15%) and interlaminar shear strength (>50%) in pultruded composites, including waste reduction and decreased manufacturing costs, are possible by manipulating the processing conditions and composition of the resin component.