This study aimed to investigate the effect of climatic factors, including temperature-humidity index (THI) and rainfall level (RFL) regarding the parity on the incidence of mastitis, milk yield, and composition in Holstein cattle in Egypt. A total of 2496 lactating cows were used from different four dairy farms located in Egypt. One hundred dairy cows were selected from each dairy herd for detection of the effect of different climatic factors and parity on milk composition and somatic cell count (SCC).The data grouped according to parity into 1, 2, and >2 parity groups. Also, the animals grouped according to THI into low (<70), medium (70-80), and high (>80) THI level and regrouped according to RFL into low (<2 mm), medium (2-5 mm), and high (>5 mm) rainfall per month. The obtained results revealed an increase in daily milk yield, milk protein%, fat %, and SCC with the increase of parity. Also, high THI significantly (p≤ 0.05) decreased daily milk yield (29.45 kg) and increased SCC (415 × 103 cell/mL). In high THI and low RFL climatic conditions, multiparous cows were prone to the occurrence of clinical mastitis compared to primiparous ones (p≤ 0.001); especially during mid and late lactation stages. In conclusion, high THI and low RFL impaired dairy performance and increased the frequency of clinical mastitis.