Breast cancer is the most prevalent cancer type and the leading cause of cancer mortality among women worldwide. Estrogen receptor-positive (ER+) breast cancer is the most common clinical subtype with an incidence rate of approximately 80% of all breast cancers. Tamoxifen is a highly effective hormonal therapy for ER-positive breast cancer patients. However, its remarkable success is hampered by de novo or acquired resistance. Despite several advances in therapy options for relapsing patients, tamoxifen resistance is still an urgent clinical problem that needs to be addressed. Therefore, there is a dire need for novel targeted therapies to confer tamoxifen resistance in ER-positive breast cancer. The architecture of Mediator complex links DNA-bound transcription factors to the general transcription machinery RNA polymerase II. Mediator kinase module is dissociable part of the Mediator complex and broadly involved in human cancers. However, the role of kinase module in tamoxifen resistance has not been investigated. In this dissertation, I deciphered the association of Mediator kinase module in tamoxifen resistance both in vitro and in vivo settings. Initially, our gene expression profiling and survival analyses revealed that Mediator subunit 13 (MED13) and cyclin-dependent kinase 8 (CDK8) were significantly higher in tamoxifen-treated patients, and this outcome strongly correlated with worsened patient survival. In vitro inhibition of either MED13 via genetic modulation or CDK8 by highly selective inhibitor, SNX631, significantly reversed tamoxifen resistance. Notably, targeting MED13 or CDK8 resulted in inhibition of HER2/mTOR signaling and triggered apoptosis. Mechanistically, we identified that inhibition of either MED13 or CDK8 combined with tamoxifen treatment reduced ErbB2 mRNA level. We further demonstrated that CDK8 post-transcriptionally controls ErbB2 level via regulating mRNA stability. Moreover, inducible silencing of MED13 in combination with tamoxifen impaired the tumor growth. Similarly, in vivo treatment of SNX631 together with tamoxifen reduced tumor growth in xenografts and prolonged the lifespan in an aggressive transgenic mouse model. These results provided insight into how transcriptional programmers MED13 and CDK8, could contribute to mediating tamoxifen resistance and added new dimension to treatment strategies for ER-positive breast cancer.