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We report here on an outbreak of mastitis caused by Streptococcus agalactiae, or group B Streptococcus, in a northern Italy (Lombardy Region) free stall dairy farm. This outbreak was unusual because it occurred in a closed dairy herd and proved to be extremely difficult to resolve even after the application of the classical control procedures, which are specifically focused on the contagious nature of S. agalactiae. In order to better understand the potential origins of the pathogen and the critical points that could impair the eradication program and to investigate the possible presence of S. agalactiae in sources outside the mammary gland, we collected 656 individual composite milk samples, 577 samples from extramammary body sites (289 rectal, 284 vaginal, and four throat samples from milking cows, dry cows, heifers, and calves), and 81 samples from the cattle environment, including the milking parlor and the barn. Twenty-two S. agalactiae isolates were obtained from lactating cows or their environment. Of these, nine were isolated from milk, two were from rectal swabs, and two were from vaginal swabs, while nine were isolated from environmental samples. Based on molecular serotyping, pilus island (PI) typing and multilocus sequence typing, all isolates belonged to serotype III, pilus type PI-1/2b, and sequence type 103 (ST103), a type previously described to have an environmental transmission cycle and a potential human origin. Once the classical mastitis control measures were supplemented with environmental hygiene measures, herd monitoring using bulk tank milk revealed no further positive results for S. agalactiae, and the outbreak was considered resolved.