Purpose To evaluate the influence of post type and mechanical aging on compression force resistance, fracture pattern, and stress distribution in weakened roots. Materials and methods Bovine roots were endodontically treated and widened—and randomly divided into 8 groups (n = 10) according to post type (prefabricated glass fiber post and customized anatomic glass fiber post, milled glass fiber post‐and‐core, and milled polyetheretherketone post‐and‐core) and mechanical aging (without and with mechanical aging). Three hundred thousand cycles of mechanical fatigue were performed and compression force resistance (N) was analyzed by two‐way ANOVA and Tukey test (α = 0.05). Fracture patterns were reported and stress distribution was analyzed by finite elements analysis. Results There was a significant effect of post type (p = 0.032) and mechanical aging (p = 0.009), but no double interaction (p = 0.879). Higher values were recorded in the milled glass fiber and polyetheretherketone post‐and‐core groups compared to the prefabricated glass fiber post groups, and no significant difference was found among anatomic glass fiber post groups and other groups. Reparable fractures were predominant in the milled glass fiber and polyetheretherketone post‐and‐core groups. Prefabricated glass fiber posts and milled polyetheretherketone post‐and‐cores showed similar stress distribution. Conclusions Post type and mechanical aging influence the compression force resistance and fracture pattern of weakened roots. Milled glass fiber and polyetheretherketone post‐and‐cores exhibited higher compression force resistance and more reparable fractures compared to prefabricated glass fiber posts. Prefabricated glass fiber posts and milled polyetheretherketone post‐and‐cores showed similar stress distribution.