Trevor Rabin was in a couple of bands, then released a few solo albums, before teaming up with Chris Squire and Alan White, which eventually became a reformation of Yes. 90125 launched Rabin to stardom.
I heard the singer from Tin Machine had a prior solo career. Some of his stuff is pretty good.
The one I could think of is almost certainly an edge case, in that I wouldn’t consider them a ‘very famous band’ and that the solo career was one of many ups and downs. Anyway, my nomination is for Kris Kristofferson and his work as part of the Highwaymen. For me it qualified in that I found the artist as part of the supergroup and then backtracked his individual work. But again, a very edge case. And now I most go listen to the song in question again because I love it and it’s stuck in my brain!
Brian Johnson doesn’t quite qualify per the OP, as he was in the moderately successful band Geordie before going solo. He later found immense fame as Bon Scott’s replacement in AC/DC, recording the second-best selling album of all time.
I suppose Adam Lambert might qualify.
As a fan of Yes, I was thinking of posting about Rabin last night, but when I did some reading, I discovered that his most notable earlier band, Rabbitt, was in existence for six years, had several albums, and a #1 hit in their native South Africa (though I think that they had little or no airplay in the U.S.). So, at least in my mind, he didn’t qualify for the OP.
Ronnie James Dio had had a solo career and fronted a couple of bands before he joined Black Sabbath.
Technically, Patty Smyth was Eddie Van Halen’s first choice, but your point is still valid.
I’d say David/Davy Jones is pretty much the model of what I was looking for. He definitely released a solo album and had a Hot 100 single, but he clearly went on to new heights with the Monkees and he’s certainly much better known as a member of them than for anything he may have done as a solo artist after the Monkees’ run ended.
I just read on Wikipedia that he was on the same episode of the Ed Sullivan show as the Beatles, promoting the stage show he was acting in, and seeing the girls go crazy for the Fab 4 wanted to become more like them and that caused him to change from acting to singing.
Some others mentioned might work as well, but I’d not ever heard of them, while I definitely heard of Davy Jones.
And whenever I talk about Davy Jones, I’m always reminded of Kingdom of Loathing doing a gag where you find Mike Nesmith’s locker.