I haven’t read them/it, but I remember seeing at least one novel by actor Rupert Everett at the library. A quick search on Amazon shows he wrote a novel called Hello, Darling, Are You Working?, an autobiography, and what looks like a book of poetry.
Stephen Fry wrote four novels (so far). I think they’re good.
Jimmy Carter wrote a Revolutionary War novel called Hornet’s Nest. It is, in a word, AWFUL.
Mussolini wrote a novel called The Cardinal’s Mistress that was a bestseller before he became Il Duce. No idea if it’s in print/in English.
Actor Stephen Collins (best known recently probably for the dad on Seventh Heaven) wrote a novel called Double Exposure. I haven’t read it personally, but it’s been heavily recommended to me.
I liked both Shopgirl and The Pleasure of My Company, and thought they worked well as audiobooks read by the author.
Yes, a good novel, but certainly not Pythonesque.
Eric Idle has also written a novel, The Road To Mars, that’s a bit more fun.
If you want to get picky, a lot of the people mentioned in this thread aren’t really non-writers, they’re just non-novelists, who are well-known for writing other things, such as screenplays, comedy routies, sketches, songs, or non-fiction. The other examples I can think of all fall into this category:
Michael J. Nelson has written a novel called Mike Nelson’s Death Rat! It’s a good humorous novel but, the title notwithstanding, not in the Mystery Science Theater 3000 vein.
Martin Gardner, best-known for writing about recreational mathematics and about science and pseudo-science, has written a novel called The Flight of Peter Fromm about religion in America. I really enjoyed it, though it’s probably not for everyone. In places it reads like a good novel; in other places it reads like a history of twentieth-century theology.
Johnny Cash wrote a novel called Man In White, about the apostle Paul. I wasn’t particularly taken by it, but apparently many people like it.
Has anybody read Jimmy Carter’s novel? From what I’ve heard, it’s not the reatest read, though not a total failure. (On preview, I see Sampiro’s opinion.)
There are plenty of examples of folks who haven’t written fiction trying their hand at it, especially in science fiction.
Carl Sagan wrote Contact. You certainly couldn’t say he wasn’t a writer – he’d written lots of popular scienvce before, not to mention many professional papers. But that’s the first fiction by him I know of.
There’s an anthology called Great Science Fiction by Scientists that includes a fictional story by mathematician Norbert Weiner called “Brain”. It’s about gangsters. There are lots of other examples in the collection (it’s all works by non-writers).
Heck, Johannes Kepler, discoverer of Kepler’s Laws of Motion (among a great many other things) wrote Somnium, in which a student of Tycho Brahe’s gets transported to the Moon (Kepler was, himself, a student of Brahe’s)
William F Buckley wrote a number of spy novels.
Does Stephen Colbert count? – I Am America and So Can Youand of course the Tech Janssen novel. 
Tony Curtis wrote at least one novel Kid Andrew Cody & Julie Sparrow. He has also had a painting displayed at the Metropolitan Museum in Manhattan.
Anthony Bourdain wrote two mysteries in the 90s well before he became famous with Kitchen Confidential. Bone in the Throat and Gone Bamboo both received glowing reviews.
Buckley was a hugely prolific writer. He wrote numerous letters, articles, columns, and many books apart from the spy novels. In fact, he became famous originally because of a book he wrote shortly after college entitled God and Man At Yale.
This from Buckley’s eulolgy, delivered by his son Christopher:
“How many words flowed from those keyboards. I went up to Yale recently to inspect his archive of papers. They total 550 linear feet. To put it in perspective, the spire of St. Patrick’s rises 300 feet above us. By some scholarly estimates, he may have written more letters than any other American in history. Add to that prodigal output: 6,000 columns, 1,500 Firing Line episodes, countless articles, over 50 books. He was working on one the day he died.”
Nick Cave wrote And the Ass Saw the Angel, a rather good Southern Gothic novel. He also wrote the screenplay for The Proposition.
Leonard Cohen wrote The Favorite Game and Beautiful Losers, although both before he came famous as a singer-songwriter.
William Morris wrote a series of novels towards the end of his life, but he’s much more famous for other things.
Disraeli, of course, wrote many novels.
G. Gordon Liddy has already been mentioned. Howard Hunt also wrote several books (significantly in Hunt’s case, he managed to have them published under a penname without being identified as the author). The works of both authors are good within the limits of the action thriller genre.
David Niven wrote a novel or two. I read one of them (Go Slowly, Come Back Quickly) which I recall enjoying.
Gene Hackman and Richard Dreyfuss have both written historical novels but I think they may have had co-authors.
I fdlipped through the Hackman one. He did have a co-author. If I recall correctly, it was a Civil War adventure story about an escape from the Andersonville POW camp. I think it was called Escape from Andersonville. I got the distinct impression from skimming through it that it had probably started life as a pitch for a movie idea, then been adapted into a ghost written novel when it couldn’t be sold.
It’s not a bad concept, though. Andersonville is an interesting subject.
Scott Turow spent 8 years as an assistant US attorney after graduating from Harvard Law School. Then as a law firm partner, started on his career as a novelist. No cite, but IIRC he writes most of his books on the train commute to and from work.
This probably does not count, but Margaret Mitchell was a newspaper writer when she wrote Gone with the Wind. Wasn’t that her only novel? If that’s true, then it was her only foray into fiction.
I read a few dozen pages of one of his retardo-chic novels – he writes like a monkey.
What kind of nard-seeking cunt editor would have published something like his turd “Sweet Valley High”-wannabe novels? I really want to know.
I was at the bookstore today and noticed novels by Glenn Beck and Joan Rivers, but on closer inspection, they both appear to have been ghostwritten.
Les Claypool wrote South of the Pumphouse. Claypool is one of the best bass players in the world. As a novelist, Claypool is one of the best bass players in the world. 
Actually I quite liked his book. Literature for the ages? Well, probly not, but I thought it was very funny (in a twisted and dark way). Some of his music can be a little “challenging” but his writing style is very straight-forward and accessible.
China Guy writes:
> Scott Turow spent 8 years as an assistant US attorney after graduating from
> Harvard Law School. Then as a law firm partner, started on his career as a
> novelist. No cite, but IIRC he writes most of his books on the train commute to
> and from work.
Siam Sam writes:
> This probably does not count, but Margaret Mitchell was a newspaper writer
> when she wrote Gone with the Wind. Wasn’t that her only novel? If that’s true,
> then it was her only foray into fiction.
People, read the title of the thread. The OP doesn’t ask for writers who had other jobs. Most writers had other jobs. It’s nearly impossible to get started in writing without having another job. This thread is about people who were famous for something other than writing who then wrote a novel.
There’s also a new novel, due to come out probably September this year.
I’ve got a signed copy of Mirror Friend, Mirror Foe, co-written by George Takei. I haven’t had the heart to read it, but it might actually be worth something.
I was going to mention Newt Gingrich. Do you have any knowledge of their writing process or are you just assuming? I highly recommend the books to any one who likes alternate history. I read the books because I am a fan of William Forstchen and they do read like his novels. I also read the Dreyfuss book and enjoyed it. I would put it on the bottom half of Turtle dove’s work but still worth the read.