The story
Mackeson's Brewery of Hythe, Kent, first released the beer in 1909 to celebrate the 240th anniversary of brewing in Hythe. They patented it and licensed it to breweries across the country, so it quickly became a national brand and attracted numerous imitators. Whitbread acquired the brand in 1929. By the 1950s it accounted for half of Whitbread's production, and was brewed at Chiswell Street in London, Stockport, Kirkstall and Hythe.
Brewing at the Hythe plant ended in 1968. The beer was then brewed at the Exchange Brewery in Sheffield. When that was closed in 1993, Whitbread moved production to their Castle Eden and Samlesbury plants. From May 1999, then Vaux brewery until production was contracted out to Young's Brewery of Wandsworth. Whitbread was purchased in 2001 by Interbrew (now Anheuser-Busch InBev). Production was then moved to Camerons Brewery of Hartlepool, before moving to Hydes Brewery in Manchester until March 2012.
From Wikipedia’s article on Mackeson Stout, shared under CC BY-SA 4.0.3