The productivity of Brassica oilseeds is severely affected by its major pest: aphids. Unavailability of resistance source within the crossable germplasms has stalled the breeding efforts to derive aphid resistant cultivars. In this study, jasmonate-mediated host defense in Indian mustard Brassica juncea (L.) Czern. was evaluated and compared with regard to its elicitation in response to mustard aphid Lipaphis erysimi (Kalt.) and the defense elicitor methyl jasmonate (MeJ). Identification of jasmonate-induced unigenes in B. juncea revealed that most are orthologous to aphid-responsive genes, identified in taxonomically diverse plant-aphid interactions. The unigenes largely represented genes related to signal transduction, response to biotic and abiotic stimuli and homeostasis of reactive oxygen species (ROS), in addition to genes related to cellular and metabolic processes involved in cell organization, biogenesis, and development. Gene expression studies revealed induction of the key jasmonate biosynthetic genes (LOX, AOC, 12-OPDR), redox genes (CAT3 and GST6), and other downstream defense genes (PAL, ELI3, MYR, and TPI) by several folds, both in response to MeJ and plant-wounding. However, interestingly aphid infestation even after 24 h did not elicit any activation of these genes. In contrast, when the jasmonate-mediated host defense was elicited by exogenous application of MeJ the treated B. juncea plants showed a strong antibiosis effect on the infesting aphids and reduced the growth of aphid populations. The level of redox enzymes CAT, APX, and SOD, involved in ROS homeostasis in defense signaling, and several defense enzymes viz. POD, PPO, and PAL, remained high in treated plants. We conclude that in B. juncea, the jasmonate activated endogenous-defense, which is not effectively activated in response to mustard aphids, has the potential to reduce population growth of mustard aphids.