A total of 20,823 Holstein × Lai Sind crossbred dairy cows from three farms in Central and East-Southern Vietnam were examined to clarify the prevalence of clinical mastitis and evaluate its effects from July to October 2022. The results indicated that clinical mastitis in cows was present at a relatively low rate (7.05%), and the high-density farms had a higher rate of clinical mastitis in cows than in other farms. In a hot, humid month, the incidence of clinical mastitis in cows (7.78%) was higher than in other months in this study, especially in the high-density farm. Cows at the first parity were clinical mastitis (32.58%) higher than at other parities, followed by cows at the second parity. As indicated, farm scale, months, and parity affected the incidence of clinical mastitis in cows in those survey farms. Clinical mastitis caused the fluctuation in milk yield, which was determined at 15.45±8.67 to 18.44±0.72 kg/cow/day; moreover, the milk yield of clinical mastitis cows in the large-scale farm showed a significant fluctuation. Among examined bacterial pathogens, Streptococcus agalactia (34.65%) was the most detected from milk samples of clinical mastitis cows, followed by Klebsiella (14.47%) and Escherichia coli (10.09%), but Staphylococcus spp. was present at the lowest rate (0.88%). The treatment followed the veterinarian’s guidelines and was adequate for clinical mastitis cows in this study within five days (79.15%). Thus, mastitis management in dairy farms is significantly required to prevent disease spreading in farms, especially in high-density herds.