The name "Seven Mile Beach" seems to have come from an old advertisement by the Jamaica Tourist Board (or their previous organizational name) that stated there was "seven miles of beach in Negril". This count included the beaches as far north as what is called "little bloody bay", Bloody Bay, Long Bay (the main long beach) as well as the beach areas located on the West End by Canoe (Mary Bay) and the small beach next to Swordfish Restaurant. In total, they added up to approximately seven miles as the advert stated.
As people tend to like to simplify things, the "seven miles of beach" soon became simply the "Seven Mile Beach" that we know today and refers to the main Negril beach of Long Bay.