Background: Inherited DNA repair gene mutations are more prevalent in men with advanced prostate cancer than previously thought, but their clinical implications are not fully understood. Here we investigated the clinical significance of germline DNA repair gene alterations in men with metastatic castration-resistant prostate cancer (mCRPC) receiving first-line novel hormonal therapy (NHT), with a particular emphasis on BRCA1/2 and ATM mutations. Methods: We interrogated 50 DNA repair genes for pathogenic or likely pathogenic germline mutations using leukocyte DNA from 172 mCRPC patients beginning treatment with first-line NHT: abiraterone or enzalutamide. We assessed the impact of germline DNA repair gene mutation status on ≥50% and ≥90% PSA response rates, PSA progression-free survival (PSA-PFS), clinical/radiologic progression-free survival (PFS), and overall survival (OS). Outcomes were adjusted using propensity score-weighted multivariable Cox regression analyses. Results: Among 172 mCRPC patients, germline mutations (in any DNA repair gene) were found in 12.8% (22/172) of men, and germline BRCA/ATM mutations were found in 5.2% (9/172) of men. In unadjusted analyses, outcomes to first-line NHT were better in men with germline BRCA/ATM mutations (vs. no mutations) with respect to ≥90% PSA responses (78% vs. 28%, P = 0.004), PSA-PFS (HR 0.47, P = 0.061), PFS (HR 0.50, P = 0.090) and OS (HR 0.28, P = 0.059). In propensity score-weighted multivariable analyses, outcomes remained superior in men with germline BRCA/ATM mutations with respect to PSA-PFS (HR 0.48, 95%CI 0.25–0.92, P = 0.027), PFS (HR 0.52, 95%CI 0.28–0.98, P = 0.044) and OS (HR 0.34, 95%CI 0.12–0.99, P = 0.048), but this was not true for men with non-BRCA/ATM germline mutations (all endpoints, P> 0.10). Conclusions: Outcomes to first line NHT appeared better in mCRPC patients harboring germline BRCA/ATM mutations (vs. no mutations), but not for patients with other non-BRCA/ATM germline mutations. These results support the hypothesis that AR may promote DNA repair, and that inhibiting AR in the context of homologous recombination deficiency may lead to synthetic lethality.