Sorry for the simultaneous post. I’m having computer problems.
There are a rare few super-villains who’ve managed to maintain secret identities. The Green Goblin, Norman Osborne, maintained his front as a wealthy industrialist for years, even after Spider-Man knew who he was. And then later on he became the head of S.H.I.E.L.D. while still occasionally making appearances in costume. His successor as Spidey’s nemesis, Roderick Kingsley, the Hobgoblin, headed a perfume company for about a decade before finally being outed in the mid-90’s.
Damn, partially ninja’d.
Mysterio, I believe, turned to crime because he’d been fired from his stuntman/special effects job.
Well yes, 90% of the supervillains out there were fired from jobs that tied into their villainy gimmick of choice. The question is what 10% of those villains were spared the pink slip. (probably more like 2% if you account for the bad guys who were gods,aliens,born into supervillainry, etc).
The Walrus is a cab driver . . . if he even counts . . .
IIRC, the Circus of Crime would perform as a regular circus for paying customers, and then, at the apex of the show, Ringmaster would use his Hypno-Hat and the other performers would go through the crowd removing the patrons of their valuables. The customers, being hypnotized, would not realize what was wrong: at least not until the Circus left town.
Baron Unterbeit was the ruler of Unterland until he was overthrown.
There’s also a Jekyll/Hyde class of villains, who have no control over their transformations, and may not even be aware of them. The Lizard and Man-Bat are both successful scientists by day, and the public is not generally aware that they sometimes turn into monsters. More recently, Mr. Negative suffered from this.
I don’t remember what Carol Ferris’s relationship with her alter ego Star Sapphire was, but she was head of an aircraft company.
Ozymandias seemed to be doing pretty well for himself.
Dr. Doom’s got a whole country to run.
Sauron had a secret identity as Annatar, Lord of Gifts, a ring maker, making him a jeweler. Darth Sidious of Star Wars was an Republic Senator.
Another one besides that one that was mentioned in the second post? 
The Sandman (The Spidey villain) had a day job for a while.
He realized that his life sucked. Every time he committed a crime, some superhero would beat him up and he’d go to jail. He finally got tired of it. The Thing showed up to arrest him and prepared for a big fight. The Sandman surrendered. Over a few beers, Sandy told the Thing his life story. The Thing said “People think that the Sandman is dead. Let him stay dead.” paid the tab and left.
After that, Sandy worked a variety of odd jobs. He worried about getting a better job. But he couldn’t so much as submit a proper W2 without alerting the authorities. Eventually, Sandy joined Silver Sable’s team as a hero.
Then some idiot writer decided to undo decades of writing and make him a villain again.
And here I was, getting ready to mention him in the other thread. 
There are a couple of other super-villain/professor tropes. There is the scientist who’s research doesn’t get funded, so takes to a life of crime to carry on his own experiments (eg Humbug), plus the Professor who gets obsessed over a female student (eg The Jackal)
I always thought Sandman struggling to reform and joining the good guys was an interesting bit of character development and was also disapointed when I saw that he went back to crime.
IIRC, Baron Zemo started out as a Nazi scientist and his motivation (which I’m pretty sure he’s told people about) is that during a battle with Captain America, Cap made his helmet/mask stick to his head with glue that wouldn’t come unstuck. Zemo invented the glue.
His motivation may have changed, of course.
Also, Emma Frost used to be the headmistress of a private school.
What sticks out in my memeory, besides Captain Boomerang (already mentioned, and presumably unable to keep his day job, as already offered) was Major William Vickers, the first Major Victory of the Force of July. A quick research just done shows me that DC has had three versions of such a character. (I had missed a lot by swearing off buying new comics (for the most part) back in '88.)
I gather that all three individuals were considered to have “legitimate” government jobs.
If the two that succeeded the original were true to form, all three could still be considered villains, by the ruthlessness of their covert actions. (Although the entire Force of July has been classed as “antagonistic superhero[es]” in the Wiki article.)