Background: Elevated Glioma-associated Oncogene Homolog-1 (Gli1) protein expression is associated with Hedgehog (Hh) pathway activation in high-grade serous ovarian cancer (HGSOC). Inhibition of Hh signaling in Gli1-overexpressing HGSOC patient-derived xenograft (PDX) inhibited tumour growth, particularly in combination with chemotherapy. Early phase HGSOC clinical trials of vismodegib, a potent Hh inhibitor (SMO inhibitor), were disappointing. We identified a HGSOC PDX harboring both Indian Hh ligand-overexpression and bi-allelic deletion of TSC1, which latter event is reported to derepress the mTOR pathway, driving non-cannonical Gli1 expression. We explored the effect of vismodegib in combination with cisplatin or the mTOR inhibitor, everolimus, in this model. Methods: A cell-line was generated from the well-characterised PDX (identity confirmed by PDX-specific p53 mutation). In vitro response to vismodegib was assessed. qRT-PCR was performed to establish Hh-ligand and Gli1 expression with/without SMO inhibition. A PDX was generated from this cell-line and randomized to in vivo treatment with cisplatin, vismodegib, everolimus or vehicle alone, or vismodegib in combination with cisplatin or everolimus. Results: The HGSOC cell-line was sensitive to vismodegib in vitro (EC50 of 3.5µM) and qRT-PCR analysis revealed down-regulation of Hh-ligand and Gli1 expression following in vitro SMO inhibition, confirming on-target vismodegib activity. In vivo treatment with vismodegib or everolimus alone did not result in reproducible in vivo efficacy. The combination of vismodegib + everolimus caused short-lived responses in 3 of 6 mice. Strikingly, in vivo treatment with vismodegib in combination with cisplatin impaired median survival (19 days) when compared with cisplatin treatment alone (43 days; p = 0.039) due to rapid tumour progression. Conclusions: Combining chemotherapy with Hh inhibition in Hh ligand-overexpressing HGSOC PDX with mTOR pathway activation may be detrimental. These findings highlight the importance of an in-depth understanding of tumour biology in order to effectively combine therapeutic approaches.