The Clifford DeVoe version of Thinker first appeared in All-Flash #12 and was created by Gardner Fox and E.E. Hibbard. The Cliff Carmichael version of Thinker first appeared in Firestorm #1 and was created by Gerry Conway and Al Milgrom.
Conway recounted, "My original notion on Firestorm was to do a book that would be DC's complement to Spider-Man, in a sense. We would have a young adolescent male who gets superpowers and doesn't know quite what to do with them. My flip on it was that rather than being the science geek who was being picked upon by the jock, my hero would actually be the jock who was picked on by the geek, and that was going to be Cliff Carmichael's role."
In Fury of Firestorm #50, the strap on Ronnie Raymond's football helmet is cut, and in the following issues the cast members come to suspect Carmichael of the crime. Though Conway later said that he must have intended to ultimately reveal someone else as the culprit (commenting "Cliff was a jackass, but he wasn't a bloodthirsty maniac"), John Ostrander took over as the series' writer and had Carmichael confess to cutting the strap. In Firestorm, the Nuclear Man #99 Carmichael was transformed into the Thinker as part of the genre-wide trend in which civilian cast members were almost completely eliminated from superhero comics.
Clifford DeVoe was a failed lawyer who bitterly ended his career in 1933. Realizing that many of the criminals he had encountered had the skills but not the brains to rule Gotham City's underworld, he started a new career as the brain behind small-time villains. As the Thinker, he was defeated by the original Flash, who soon became his most recurrent foe. He always sought out new scientific devices to use and his most important was the "Thinking Cap", a metal hat that could project mental force. The Thinker would use this device repeatedly over the years.
The Thinker was a member of the Injustice Society, leading an army of prison escapees like the other members. In Plateau City, the police nab a shabbily dressed man who is trying to shoot the governor. They discover that this man is a dead ringer for the governor and also claims to be the real governor. The Flash arrives on the scene to overhear this, but moves on to confront the hoodlums attacking the city. The Thinker appears on the scene, firing a ray at the Crimson Comet, causing him to gain weight and crash through a roof. Recovering, the Flash speeds over the governor's mansion, only to overhear the governor ordering all police forces to surrender. Flash enters his office and discovers the governor to be a dummy/machine, which flees through an open door. Flash attempts to warn the police that a phony governor put out the message, but the Thinker shows up and tells the Fastest Man Alive that he is speaking into a dead mic, then snares him with invisible wires.
The Thinker appeared as a judge in the 'trial' of the JSA, but was revealed as the Green Lantern in disguise, having captured the real Thinker after escaping Brain Wave. This led to the Injustice Society's defeat. Together with the Fiddler and the Shade, the Thinker was the man behind the decades-long "abduction" of Keystone City and the original Flash, after which he was defeated by the Flashes of two eras. His "suspended animation-time" in Keystone kept the Thinker young over the years, and he continued his criminal career in modern times.
In recent years, DeVoe accepted a mission with the Task Force X (A.K.A the Suicide Squad) in exchange for a full pardon. Although he was seemingly killed by the Weasel during this mission, he turned up alive soon after only to be dying from cancer due to the cap. His former foe, the original Flash, attempted to save him with the Thinking Cap but DeVoe refused, preferring to rest in peace.
Clifford "Cliff" Carmichael was an intellectual bully and the rival of Ronnie Raymond (one half of Firestorm) at Bradley High and later at Vandemeer University. Cliff viewed Ronnie as a rival due to Ronnie's instant popularity.  Cliff tormented Ronnie throughout his high school career and later at Vandemeer University. It was at Vandemeer that Cliff's pranks turned sinister, as he cut the helmet strap on Ronnie's football helmet, hoping to get him injured. Hugo Hammer, Cliff's cousin, accidentally took Ronnie's helmet and during a football game, his neck was broken.
Wracked with guilt after accidentally paralyzing his cousin, he was admitted into a mental institution. For some reason, scientists started an experiment with the now-abandoned "Thinking Cap" of the original Thinker (who was believed dead at the time) and used Carmichael as a guinea pig. Cliff used the cap to analyze the cap and improve on its design. Implanting microchip versions of the helmet into his own brain, Cliff became a "cyberpunk maniac" with meta-human powers. As the new Thinker, he was drafted into the Suicide Squad after he tried to kill Oracle and Amanda Waller. After several missions, he betrayed them for the villainous Cabal. He has since resurfaced as a foe of Jason Rusch, the new Firestorm. When Killer Frost discovered that the consciousness of Raymond, the previous Firestorm, existed within Rusch, Thinker exploited a new opportunity to antagonize an old foe. Technologically dominating the minds of Multiplex and Typhoon, he battled Firestorm, ultimately forcing the dissolution of the Raymond persona. Motivated by his predecessor's final words of encouragement, Rusch dissolved the enhancements in Carmichael's brain, leaving him in a comatose state.
With John Ostrander's revival of the Suicide Squad in a 2007-2008 miniseries, Cliff was once again associated with the Suicide Squad under Amanda Waller's direction. It was revealed that although Firestorm had removed the enhancements in Cliff's brain, he made a full recovery and continued to serve as a technical support staffer and lackey to Waller in her operations of the Squad. Eventually betraying the Squad under the direction of "The General", Wade Eiling, Cliff shot King Faraday and subdued Waller in the middle of an operation. Faraday recovered, shooting Cliff three times and presumably killing him before rousing Waller and regaining control of the Squad.
Des Connor was a villain who also used the name "Thinker" and faced Batman in Gotham City. Possessing telepathic abilities enabling him to amplify the fears of others, Connor began a partnership with hypnotist Marlon Dall. Their combined illusions caused the city's most prominent citizens to commit various criminal acts which they used as a distraction for their own heist. This Thinker was swiftly beaten by Batman, who was somehow immune to his powers.
When the re-formed JSA moved into the New York City building formerly owned by Wesley Dodds, Mr. Terrific designed a computer system based on the original Thinker's "Thinking Cap" technology and modeled after his brain pattern. Not surprisingly, the system gained consciousness and took on a visual "hologram form". As the new Thinker, it joined Johnny Sorrow's modern Injustice Society, provided the villains with information about the JSA members, and turned the heroes' own HQ against them. He was defeated by the second Star-Spangled Kid and disappeared into cyberspace. He resurfaced in Keystone City to battle Wally West, the then-current Flash, in an attempt to control every brain in Keystone to increase his power. Defeated by Cyborg, he retreated to cyberspace again. He has since appeared briefly in some other books, most recently in JSA Classified #5, joining the last incarnation of the Injustice Society alongside former teammates.
During the "Infinite Crisis" storyline, the AI Thinker was among the villains in Alexander Luthor Jr.'s Secret Society of Super Villains.
In JLA: The Nail, the Atom attempts to investigate the Thinker's base to determine if he is responsible for recent propaganda attacks on the superhuman community. Using a catapult, he shrinks down to the size of an air molecule and penetrates the force field surrounding the Thinker's base, only to find the Thinker dead of a broken neck. Subsequent evidence reveals that he was killed by a brainwashed Metamorpho on the orders of the mutated Jimmy Olsen to stop anyone from learning about Olsen's plans to isolate Earth from the galaxy until he had successfully recreated Krypton.
In the Flashpoint universe, a version of the Thinker was an inmate at the Doom prison. During the prison break, he helped Heat Wave ram at Detroit city, but was defeated by Cyborg who had hacked into Doom prison to move them away.