Anti-Müllerian hormone (AMH), produced only by granulosa cells, is a biomarker for ovarian reserve in sheep. This study compared AMH levels in two Slovenian breeds: Istrska pramenka (IP), a seasonal breeder, and Jezersko–Solčava (JS), which reproduces year-round. Blood serum samples from 78 sheep were analyzed during the estrus period using an ovine AMH ELISA. JS sheep were grouped by age (1–3, 4–6, ≥ 7 years), while the majority of IP sheep were ≥7 years. AMH levels differed significantly between breeds (p < 0.001), with JS sheep showing higher concentrations. No age-related differences were found within the JS breed (p = 0.752), but JS sheep ≥ 7 years had higher AMH than IP sheep of the same age (p < 0.001). AMH levels were also higher in ewes lambing two lambs (p < 0.001) and were positively correlated with litter size, particularly in the JS sheep ≥ 7 years (p < 0.001). These findings suggest AMH is a useful fertility marker in sheep, although breed-specific reference values are essential for an accurate interpretation. The study highlights the influence of breed and age on AMH levels and its potential role in predicting reproductive performance.