McMullen's, known locally as Mac's, is a regional brewery founded in 1827 in Hertford, England. The brewery expanded during the second half of the 19th century by purchasing other breweries and their associated pubs. In 1902 it was the second largest brewery in Hertfordshire. The brewery has occupied several different sites in Hertford and moved to its current location in 1891. There have been several breweries on this site and the current one opened in 2006. As of 2021, members of the 6th generation of the McMullen family are still involved with the business.