The molecular weight distribution (MWD) has a significant impact on the properties of polymeric materials, however, the characterisation of polymer MWDs has been limited to statistical parameters such as the number-average molecular weight (Mn) and dispersity (Đ). These parameters do not fully express the features of polymer MWDs, thus limiting the ability to rationally design complex polymeric materials with tailored MWDs. Herein, a platform for the design and synthesis of arbitrary polymer MWDs is developed and experimentally validated. The platform is based on the description of polymer MWDs as a mathematical function, rather than individual statistical parameters. As such, the complete shape of arbitrary polymer MWDs can be designed using a developed software. The software requires only a calibration using model monomodal MWDs directly obtained from GPC to design theoretical MWD. Using this platform in conjunction with a flow-mediated polymerisation approach, a range of arbitrarily shaped polymer MWDs were successfully designed and prepared. Finally, complex triblock copolymer mixtures with tailored compositions and overall MWD were fabricated via a one-pass flow-mediated polymerisation using this computer-guided approach.