Abstract Context While age-related changes in menstrual cycle length are well known, it is unclear whether anti-Müllerian (AMH) or other ovarian reserve biomarkers have a direct association with cycle length. Objective To determine the association between biomarkers of ovarian reserve and menstrual cycle length. Methods Secondary analysis using data from time to conceive (TTC), a prospective time to pregnancy cohort study. The age-independent association between cycle length and biomarkers of ovarian reserve was analyzed using linear mixed and marginal models. Study participants were TTC-enrolled women aged 30-44 years with no history of infertility who were attempting to conceive for <3 months were enrolled. Serum AMH, follicle-stimulating hormone, and inhibin B levels were measured on cycle day 2, 3, or 4. Participants recorded daily menstrual cycle data for ≤4 months. The primary outcome was menstrual cycle length; follicular and luteal phase lengths were secondary outcomes. Results Multivariable analysis included 1880 cycles from 632 women. Compared with AMH levels of 1.6 to 3.4 ng/mL, women with AMH <1.6 ng/mL had cycles and follicular phases that were 0.98 (95% CI –1.46, –0.50) and 1.58 days shorter (95% CI –2.53, –0.63), respectively, while women with AMH >8 ng/mL had cycles that were 2.15 days longer (95% CI 1.46, 2.83), follicular phases that were 2 days longer (95% CI 0.77, 3.24), and luteal phases that were 1.80 days longer (95% CI 0.71, 2.88). Conclusion Increasing AMH levels are associated with longer menstrual cycles due to both a lengthening of the follicular and the luteal phase independent of age.