He was, of course, a mad scientist, able to invent all sorts of bizarre super weapons in his attempts to destroy Superman and/or conquer the world. It was his intelligence--he may very well be the smartest man in the world--and the imaginative albeit evil inventions he uses that made him a villain who could believably take on the Man of Steel.
[Later attempts to retcon him into an evil businessman were never completely satisfying--though Clancy Brown's superb voice work on the 1990s Superman animated series still made him an extremely effective villain.]
Before long, Lex was being drawn as a bald guy--probably to give him a little more visual distinctiveness. But it was a full two decades after his first appearance that we finally find out why he's bald.
Jerry Siegel, who created both Superman and Lex, was back with DC Comics in 1960 and he--appropriately enough--finally explained Lex's lack of hair AND his hatred of Superman.
"How Luthor Met Superboy" was published in Adventure Comics #271. It's here we find out that a teenaged Lex, with a full head of red hair and dreams of becoming a scientist, lived in Smallville at the same time Clark Kent did.
This is a pretty cool situation for Lex, because he has a serious case of hero-worship for Superboy. When he gets a chance to save Superboy from some kryptonite, the two become friends. Superboy builds Lex a modern lab.
Lex quickly shows his scientific smarts by creating artificial life. But a lab accident causes a fire and, when Superboy blows the fire out, some toxic chemical fumes cause Lex's hair to fall out.
That sends Lex off on a quick trip to Crazy Town. He vows eternal hatred of Superboy. The situation is exacerbated when Lex tries several experiments to improve crop production around Smallville. These go awry, forcing Superboy's intervention. Lex, in his current frame-of-mind, assumes the Boy of Steel is actually sabotaging his efforts.
So by the time Lex and Clark are adults, the now-mad scientist is one of the world's most nefarious super-villains. He was pretty much an out-and-out villain, but there were occasional (and relatively successful) attempts to humanize him.
For instance, in Superman's Girlfriend, Lois Lane #23 (February 1961), we discover that Lex had a sister who was still a baby when he turned evil. Her parents changed their name and Lena Thorul never knew who her brother was. But Lex knows about her and even set aside his vendetta to team up with Superman when Lena needed to be rescued from the Bottle City of Kandor. (That happens in Action Comics 297 and 298-two issues I'd love to review, but I sadly do not own them or reprints of them.)
In another instance, he shows regret when he inadvertently stops Superboy from preventing Abraham Lincoln's assassination.
But it's in Superman #164 (October 1963) that Lex receives his most effective bit of character development. I've actually written about this story before in an old post about Edmond Hamilton, but it's worth revisiting.
After escaping from prison again (because some doofus of a warden let him use the machine shop!), Lex challenges Superman to a one-on-one fight on a planet with a red sun, where the Kryptonian will have no powers. Figuring that the world will lose confidence in him if he refuses, Superman accepts.
It's a great story. On top of Hamilton's usual skill in using comic book logic to built an imaginative but internally consistent story, this view of Lex really does give him some depth and generates a bit of honestly-earned sympathy for him. Heck, Lex is such a hero to these people that they rename the plant Lexor. He even marries one of the inhabitants--a pretty lady named Adora. They later have a son.
1983 issue of Action Comics, this has some rather tragic consequences for the entire planet of Lexor.
We have a couple more villains to cover, but eventually, we'll visit one of the "imaginary" (out of continuity) tales from the era. That one involves Lex at his most villainous and it my personal favorite of those imaginary yarns.