Who was the first major fictional detective to use PCs in his/her investigations? The earliest one I can remember was V. I. Warshawski (at least in her movie), and “use” is a slight exaggeration. The golden age of hard boiled detectives was a good 40-50 years before the PC boom, but someone had to be notably the first. (And no, I don’t care about Batman’s Bat-Computer; it was a fantasy device in the 40s and 50s and kind of irrelevant to what I’m looking for. Superheroes need not apply; I’m looking for mystery novels here.)
Your thread title says “computers” but your text says “PCs,” so I’m not sure if this is what you have in mind. Lots of detective TV series of late 1960s and early 1970s feature searches on mainframe or midrange computers, complete with requisite clips of punch cards being spat out or spinning reels of magnetic tape, with all the necessary blinking lights and miscellaneous beeps and boops. Typically the main character doesn’t use the computer directly, but gets somebody else to do it for them. I’m sure I saw something of the sort in a first-season episode of The Rockford Files (1974-75) and I also seem to recall it was a fairly frequent element of the original Hawaii Five-O (1968-1980).
It was a big part of the first season of Mannix in 1967, where he worked for a high-tech detective firm that used computers to sort through data. Mannix himself was skeptical of them, which was the hook to allowed him to go off on his own in the second season (the real reason was that Lucille Ball, head of Desilu, didn’t think the average viewer could related to the computer stuff).
The company Mannix worked for was called Intertech. His boss was, IIRC, Joseph Campanella.
There was also an episode of The Fugitive around this time in which Lt Girard (and colleagues) used a computer to predict Richard Kimball’s next move(s).
Yep. Didn’t 1960’s Batman have a computer in the Batcave?
In the late 60s, TV writers would use a vague notion of “computers” as kind of a plot device. The computer in The Fugitive wasn’t particularly different from the computer in Star Trek in this regard. I think Lucille Ball had pretty good instincts. But 20-25 years on, a “typical” worker (someone not developing software for NASA or the military, but instead someone who sold insurance or tracked down the availability of car parts, for example) might have an early PC on his desk and a pretty good idea how to use it. A detective at this stage of computer culture might find computer networks useful for accessing public records or Usenet newsgroups. So: Who would be the first TV or mystery novel detective to keep a personal computer on his desk and use it in a fairly realistic manner?
If The Cuckoo’s Egg were fiction instead of “true crime,” the answer would be Cliff Stoll, but I am looking for a fictional detective.
Computers are used in Foucault’s Pendulum.
Let me explain why I’m asking, it might narrow down the possible answers.
I’m making a PowerPoint presentation for my students, English language learners in Korea. It’s about famous fictional detectives and why each was different from the ones who came before: Sherlock Holmes, Sam Spade, Mike Hammer, Nero Wolfe, Miss Marple, Nancy Drew, Encyclopedia Brown, etc. Since personal computers are a big part of these kids’ lives, I want to be able to have a detective in this line who was the “first” to use computers in a big way, someone who uses PCs the way Holmes uses deductive reasoning or the way Nero Wolfe uses Archie Goodwin to do all the legwork, or the way Adrian Monk uses OCD. Who would be a good candidate for this?
Hmmmmm. You mean like in 24, NCIS, and Criminal Minds? Those shows have been around for almost 20 years now.
There was Sandra Bullock in The Net back in 1995.
Going farther back, there was Max Headroom in the '80s.
Oh, and let’s not forget Automan.*
How closely these correspond to your desire for accuracy and believability, I’ll let you decide.
*I’m joking, of course! 
They’re a little recent for my purposes.
Sandra Bullock in The Net might be perfect.
I kind of thought someone more familiar with the mystery novel genre would know of a more clear-cut example I could use, although it makes perfect sense that as detectives incorporate higher tech into their work, mystery writers would do the same, and would prefer electronic media to disseminate it.
How about Sneakers, from 1992? They’re security specialists, though, not detectives.
I don’t know - she was the victim, not the detective.
Yes, but she got out of her plight and nailed the bad guys at the end with her computer(s).
That’s 1995. Whoopi Goldberg beat that by nine years with Jumpin’ Jack Flash.
ETA: Granted, that was spies and not detectives.
And Blade Runner is even earlier.
2019? 
Nothing to add, except I think from the OP’s reasons for asking he’s looking for examples set in the contemporary timeframe to the publication/release.
OB
It’s a complicated question you want a simple answer to. I doubt you’ll get one except as an example.
A random google gives a 1979 uk show where the now radio dj host, and detective, is a former computer expert. So not, but he’s used computers, but not for solving things.
So “first” will go way back before that. And “computer” will not be in the same context of any normal PC nowadays (a drama computer, like Star Trek). And “computer to solve” will likely actually not be a computer, it will likely to actually be a “database search using a computer” which is barely any different from someone going through a big binder of files on a case.
If that counts, War Games beats it 
Don’t want Batman, you say. “Superheroes need not apply,” you say.
How about the TRS-80 Computer Whiz Kids, then?
I respect comic books and superheroes as much as anybody on this board, but this just isn’t a comic book question. Anyway, the first superhero to use a PC in a believable manner was Foolkiller.
A lot of the contenders aren’t “detectives” per se, but then, neither are Jessica Fletcher or the Hardy Boys. Sandra Bullock’s character is at least as qualified as they are, and also as disqualified.
I hesitate to use anything from before about 1990, just because most writers used computers as plot elements about the same way they’d use the Magic Mirror or a Ouija board. I may make an exception for Mannix, since it was his original gimmick, but my own recollection of the series was, he got dangled out of high windows kind of a lot. His trademarks were white knuckles and gritted teeth. And, as was pointed out, he never actually used the computers himself.