Plutella xylostella (L.), a menacing threat to cruciferous crops, exhibits cross-resistance to various chemical agents. The exploration of plant-derived insecticides emerges as an intervention strategy for the successful management of P. xylostella. Millettia pachyloba Drake is renowned as a traditional remedy for diverse health issues and has insecticidal properties. Experimental investigations in both laboratory and greenhouse settings utilized M. pachyloba extract (EMPE) at concentrations ranging from 2 to 10% (w/v). The objectives included inducing toxicity and controlling P. xylostella larvae effectively, assessing nutritional impacts through parameters like relative consumption rate (RCR), relative growth rate of larvae (RGR), efficiency of conversion of digested food (ECD), efficiency of conversion of ingested food (ECI), approximate digestibility (AD), and assimilation ratio (AR), and evaluating leaf damage inflicted by P. xylostella larvae on Brassica juncea. In the laboratory, the application of EMPE on P. xylostella larvae and pupae for 24, 48, and 72 hours yielded markedly higher mortality rates than the water-treated control (p < 0.05). Significant reductions in RGR, RCR, ECD, ECI, AD, and AR were evident throughout the larval stage (p < 0.05). In the greenhouse, EMPE treatments demonstrated notable differences from the water control treatment. On the 15th day of treatment, the EMPE treatment at 10% (w/v) exhibited the highest mortality rate (p < 0.05). Significantly reduced leaf damage was observed with EMPE treatments, displaying an inverse correlation with escalating concentrations. Particularly, the highest enhancement across all surveyed parameters was observed in the EMPE 10% (w/v) treatment, which was comparable to the positive control with fipronil (p > 0.05). Noteworthy differences in damage reduction percentage (DRP) were identified between EMPE contact treatments and the water control group (p < 0.05), indicating the promising potential of Millettia pachyloba extract for pest control.