The origin of the Defenders can be traced back to two crossover story arcs by Roy Thomas prior to the official founding of the team. The first, in Doctor Strange #183 (November 1969), Sub-Mariner #22 (February 1970), and The Incredible Hulk #126 (April 1970) occurred when the Doctor Strange series was cancelled and the storyline was completed in the other series. Dr. Strange teams with Sub-Mariner, then Hulk to protect the Earth from invasion by Lovecraftian inter-planetary beings known as the Undying Ones. Barbara Norriss, later the host of the Valkyrie, first appears in this story. In the second arc (featured in Sub-Mariner #34 and #35, February and March 1971), Namor enlists the aid of the Silver Surfer and the Hulk to stop a potentially devastating weather control experiment (and to inadvertently free a small island nation from a dictator) and face the Avengers. These two story arcs were reprinted in the first Essential Defenders volume.

The Defenders first appeared as a feature in Marvel Feature #1 (December, 1971), where the founding members gathered to battle the alien techno-wizard Yandroth and decided to remain as a team. They soon got their own eponymous series where they first battled the Undying Ones.

The best-known and most prominent members of the Defenders are Doctor Strange, the Hulk, Namor the Sub-Mariner, the Silver Surfer, Nighthawk, Valkyrie, and Hellcat. Though the team was founded by Strange, Namor, and the Hulk, the others are generally considered to be core members of the team. A large number of other heroes worked alongside the team in its original incarnation, with a number becoming "official" members. Other notable members include Hawkeye, Devil-Slayer, Son of Satan, Clea, Moondragon, the Gargoyle, the Beast, Iceman, and The Angel.

As of issue #125, Defenders was retitled to New Defenders as the "big four" (Doctor Strange, The Silver Surfer, The Hulk and Namor) were forced to leave the team. This was the result of an alien prophecy that stated that these four, operating as a group, would be responsible for destroying the world. The remaining group of Defenders finally disbanded in New Defenders #152, as a number of the members had apparently died and several others left the team to join X-Factor. Several of these seemingly-deceased members later returned in issues of Solo Avengers, in Strange Tales vol. 2 #5-7, followed by issues #3-4 of the relaunched Doctor Strange, Sorcerer Supreme series.