Mr. Freeze used to be one of these, with no real backstory, just the whole freezing schtick. Then came Batman: The Animated Series which gave him a chillingly tragic backstory and motivation, turning him more into a Woobie, Destroyer of Worlds. This new version of the character was Retconned into the main DCU.
Scarecrow is an expert psychologist who creates fear gas that preys on the target's most deeply seeded phobias.
Poison Ivy is a botanist that became a plant-controlling metahuman following a freak accident. While she also uses her feminine wiles as a weapon, she is still very much a mad scientist due to her disturbed unhinged love for plants outweighing humans.
Hugo Strange is the most archetypal mad scientist. He's an expert in everything from chemistry and genetics to psychology, and uses it for evil. And with his typical wardrobe of lab coat and goggles, he dresses the part as well.
"Monsters in the Closet", in the Batman: Black and White anthology series, features a one-off appearance by a mad scientist who kidnaps people and turns them into fish-person hybrids. One of them gets him in the end.
In The Attack of the Annihilator, Kenneth Anderson is turned into a psychic mutant as running experiments on alien energies, and decides to destroy Gotham and rebuild it in his image.
The Joker is usually portrayed as something of a Mad Chemist, being able to invent and manufacture deadly chemical weapons, like his signature Joker Venom.
Professor Merson, an American scientist working for Germany, was the source of countless Nazi superweapons (including the War Wheel) in the 1982 Blackhawk revival. It can be hard to tell — Blackhawk seems to feature gadgetry unbelievably ludicrous enough to fall under mad science in every issue, and frequently right on the cover.
Doctor Sivana and his family are similarly the archenemies of Captain Marvel and friends. Sivana, in particular, may be the Ur-Example in comic books, predating Lex Luthor by several months. He's a five-foot-tall gnome of a man with a chrome dome, huge Scary Shiny Glasses, and more often than not a white lab coat. His stated goals (in no particular order): To become Rightful Ruler of the Universe in fact as well as in name; to spread evil, cruelty, and nastiness throughout the cosmos; and to humiliate, discredit, and ultimately KILL CAPTAIN MARVEL!. The original version of the character was actually a benevolent man who was ruined by being rejected by the scientific community for his ideas. When his wife died, he blamed the world and turned into the crackpot we love to hate. This was the Pre-Crisis origin, the current version seems to always have been mean.
In Thunderworld #1, the Dr. Sivana of Earth-5 takes exception to being called 'mad'. He sees himself more as a radical genius.
A heroic Mad Scientist in The DCU is Doctor Magnus, creator of the Metal Men. When off his meds he hallucinates and can accidentally create sentient robots without fully meaning to.
Most of incarnations of Lex Luthor. In the years since, he's also been a Corrupt Corporate Executive and a villainous politician. He did return to being a mad scientist in Post-Crisis after the Superman: Up, Up and Away! storyline, where Clark Kent brings his Villain with Good Publicity status down through journalism. Most interpretations from the late 90's onward merge the corporate and scientist portrayals into one. Nowadays, Luthor is portrayed as building his company on his brilliant inventions, and he still gets actively involved in LexCorp's projects.
Bertron, the alien scientist in Superman/Doomsday: Hunter/Prey, who was responsible for the creation of Doomsday. He subjected the creature to the harsh environment of Krypton in the distant past, cloning it again and again every time it was destroyed either by the environment or by its resident creatures. Over the years it evolved into its current form, which would later be known as Doomsday. However, once the creature had reached the point where nothing on the planet could withstand him, he turned his attention on his creators and killed them and Bertron in the process.
In 2015, Jor-El was turned into one. Rogol Zaar, a genetically-engineered, immensely powerful and ferocious creature revealed to be responsible for the destruction of Krypton, was later revealed to have been created by Jor-El in Superman (Brian Michael Bendis), who "set him loose into the galaxy" to do his bidding as his mercenary. All to further the agenda of the Circle, a secret organization of galactic leaders who pulled the strings from behind the scenes.
The Ultra-Humanite (arguably comics' first supervillain) who actually transferred his brain from the standard baldie-in-a-labcoat mad scientist's body into that of a beautiful woman. He was only another Mad Scientist in the Golden Age comics, but in the series The Golden Age, he becomes the arch-villain, posing as a hero and getting the medal of honor. He saved Hitler's brain, too. And put it in an invincible super-body.
Karmang from Superman vs. Shazam!, who previously used his advanced science to make himself immortal, builds two devices to blow two Earths up.
A large number of the Pre-Crisis Phantom Zone prisoners are scientists who got people hurt or caused environmental harm while testing various new inventions (a rocket system, new chemical processes, a cyrosleep device, an immortality potion, labor robots, etc.).
Foe Byrna Brilyant is a scientist with no compulsions about stealing the work of others and then combining and modifying it for profit, generally by hiring herself out in her Humongous Mecha capable of causing winter storms or holding things for ransom...with winter storms.
Paula von Gunther showed enormous promise as a horrific mad scientist focused on psychiatry and meddling with the human mind, along with figuring out a way to make herself invisible among other things while she was working for the Nazis. After it was revealed she was being forced to do so and having her daughter they'd been holding hostage rescued she proves an even more impressive and varied scientist steadfastly allied with Diana and the Allies for saving Gerta. The underling mindset seems to be genetic as her daughter Gerta, well meaning as she may be, accidentally creates a number of super-powered villains over the years.
Queen Atomia is a scientist who has figured out how to make a gas that shrinks those who breathe it to a microscopic scale, has created two machines that turn humans into her near mindless slaves, and has a great interest in the potential uses for nuclear energy, and bombs.
In Wonder Woman (1987) Veronica Cale is a scientist and CEO who thinks it's a good idea to nab Doctor Psycho from prison in order to do experiments on how his mental powers work, and purchase the newest Silver Swan to learn more about the modifications done to the poor girls body. She also intends to find a way to mentally control both of the aforementioned villains to turn them into her own lackeys.
