Abstract: PLA and PBS are considered promising and sustainable biodegradable fibres that can be used as a replacement for the most dominantly used polyester fibre in the textile industry. To be fully grown up as textile fibres, the dyeability of these fibres is a top requirement. The dyeing mechanism of PLA and PBS differs from polyester due to their mechanical and physical properties. Polyester fibre has an affinity towards a few selective synthetic dyes, such as disperse dyes, and PLA can be dyed with these dyes using the high-temperature exhaust dyeing method. However, PLA and PBS fibres evince dyeability at a lower temperature than polyester and disperse dyes as higher dyeing temperatures may lead to fibre hydrolysis or even melting. Moreover, the toxicity and synthetic microplastic nature of these dyes can cause biodegradability issues in dyed PLA and PBS end products. With the growing concern for sustainable dyeing practices in the textile industry, bio-based dyes for PLA and PBS dyeing could be a safe solution for humans and the environment. In this research, five bio-based dyes—madder extract, weld extract, catechu colour, hemox colour, and cochineal dyes—were explored through the exhaust dyeing method with mordant pre-treatment. These bio-based dyes were selected based on the presence of colouring compounds in the dye materials and their environmentally friendly sourcing. The most suitable dyes were evaluated based on dye exhaustion, colour strength (K/S value), and fastness properties such as light, washing, and rubbing. Pre-treatment was conducted at 85°C for 45 minutes using mordants such as potassium alum and tannin. Dyeing was also performed at 85°C for 45 minutes, followed by after-treatment at 50°C. With pre-treatment, the washing and rubbing fastness of dyed PLA and PBS improved compared to unmordanted textiles. However, dyed PBS textiles showed significantly lower light fastness than PLA, and the mordant had minimal influence on this result. Among the five dyes, madder extract demonstrated comparatively good affinity towards PLA and PBS, with a higher exhaustion rate and K/S value than other selected natural dyes. The colour strength of PLA and PBS fibres varied, with PBS showing a higher dye exhaustion rate than PLA. To determine the optimized exhaust dye bath conditions for both fibres, various dyeing parameters such as temperature, pH, time, and dye concentration were analysed. Through this analysis, it was found that dyeing with 2% dye WOF (weight of the fabric) at 85°C for 45 minutes provides the optimum dyeing condition for PLA, while for PBS, 60°C is preferable. Additionally, pH influenced colour wavelength changes for natural dyes on PLA and PBS. As part of the biodegradability impact analysis, PLA and PBS dyed with Disperse Blue 56 and natural dyes were tested in a lab-scale experiment, where synthetic dyed textiles showed noticeable effects. Overall, this research found that dyeing PLA and PBS with natural dyes offers environmental benefits, including enhanced biodegradability and toxic-free dyeing effluent. Selective natural dyes, such as madder extract, contain suitable chromophore compounds that show affinity towards PLA and PBS. However, fastness properties improved when potassium alum was used as a mordant in pre-treatment. It was suggested that applying a UV stabilizer treatment could further enhance light fastness.