Original form of what might be known today as a MUD is developed on the PLATO system.
The first known MUD is started by Roy Trubshaw and Richard Bartle at Essex University on a DEC PDP-10 in the UK.
Mark Jacobs created and deployed a commercial gaming site, Gamers World. The site featured two games coded and designed by Jacobs, a MUD called Aradath (which was later renamed, upgraded and ported to GEnie as Dragon's Gate) and a 4X sci-fiction game called Galaxy which was also ported to GEnie. At its peak the site had about 100 monthly subscribers to both Aradath and Galaxy and it was shut down in the late 80s.
TinyMUD began to allow players to easily participate in creating the online environment, as well as playing in it. The TinyMUD code spawned a number of descendants, including TinyMUCK and TinyMUSH, which added more sophisticated programmability.
The release of DikuMUD, which was inspired by AberMUD, lead to a virtual explosion of hack-n-slash MUDs based upon its code.