Mastitis caused by Escherichia coli and Streptococcus uberis remains one of the leading causes of antimicrobial use in dairy cattle, contributing to resistance development and economic losses. This study evaluated the therapeutic potential of the natural flavonoid morin in clinical mastitis in dairy cows. The in vitro antimicrobial activity of morin (1–3% w/v) was assessed by disk diffusion, and the 3% formulation was selected for an in vivo field trial. Seventy-two Holstein–Friesian cows with mastitis caused by E. coli or S. uberis were randomly assigned to one of three intramammary treatments: 3% morin, phosphate-buffered saline, or an antibiotic, serving as a positive control. Somatic cell count (SCC) and clinical scores were monitored for seven days. In E. coli infections, morin significantly reduced somatic cell scores at 144 h and 168 h and improved clinical scores from 48 h onward, showing efficacy comparable to antibiotics. In S. uberis mastitis, morin induced clinical improvement at 96–168 h but resulted in slower and smaller SCC reduction than antibiotic control therapy. Phosphate-buffered saline produced no significant changes. These results indicate that morin exerts anti-inflammatory and supportive effects in bovine mastitis, particularly in Gram-negative infections, but is less effective against S. uberis. Further studies on pharmacokinetics, bacteriological cure rates, and optimized formulations are warranted to confirm its clinical utility.