Poly-hydroxy-butyrate (PHB) bioplastic resin can be made directly from atmospheric CO₂ using cyanobacteria. However, higher PHB productivities are required before large-scale production is economically viable. Random mutagenesis offers a way to create new production strains with increased PHB yields and increased biomass densities without complex technical manipulation associated with genetically modified organisms. This study used staining with lipid fluorescent dye (BODIPY 493/593) and fluorescence-activated cell sorting (FACS) to select high lipid content mutants and followed this with a well plate growth screen. Thirteen mutants were selected for flask cultivation and two strains produced significantly higher PHB yields (29% and 26% higher than wild type), biomass accumulation (36% and 33% higher than wild type) and volumetric PHB density (75% and 67% higher than wild type). The maximum PHB yielding strain (% dcw) was 12.0%, which was 43% higher than the wild type (8.3% in this study). The highest volumetric PHB density was 18.8 mg PHB/L compared to 10.7 mg PHB/L by the wild type. To develop cyanobacterial strain with higher PHB productivities, the combination of random chemical mutagenesis and FACS holds great potential to promote cyanobacteria bioplastic production becoming economically viable.