The current study aimed to evaluate aflatoxin M1 (AFM1) contamination in a total number of 240 samples collected during four consecutive seasons (from January to November 2022) from two western Iranian provinces (Ilam and Lorestan). AFM1 was determined by high-performance liquid chromatography with a fluorescence detector after the toxin extraction with immunoaffinity columns. The results showed 100% of AFM1 contamination in all the surveyed matrices. Considering the seasonal variability, AFM1 mean values in milk, cheese, and yoghurt samples collected during the summer were significantly higher than those detected in samples from other seasons (p < 0.01), and the lowest concentrations were observed in samples collected during the winter. AFM1 levels in cheese samples appeared safe, but an alarming rate of 46.6% of milk samples and 100% of the yoghurt samples exceeded the maximum permissible limit set by the European Union (50 ng/kg). Moreover, while the deterministic risk assessment demonstrated no risk of developing liver cancer in adults through the consumption of these dairy products, it showed significant concerns for children consuming milk and yoghurt (hazard index above 1). Overall, the results of this research indicate the necessity of a periodical monitoring program for AFM1 in milk and other dairy products.