Examples:
George Benson
Karen Carpenter
Nat "King" Cole
Louis Armstrong
Others?
Diana Krall
Allison Kraus.
Phil Collins
Anonymous
Sylvia McNair
Robert Cray
Billy Preston
Nina Simone
Chet Baker.
Blossom Dearie did both, wound up mostly singing, but I don't know how she started.
The great singers Carmen McRae and Sarah Vaughan -- both pianists.
Harry Connick, Jr.
Herb Alpert?
Sheila E.
Half of the Motown stable. They had to work their way up from writing/playing some instrument/back-up vocals to stars.
Barrett Strong - piano
Smokey - songwriter
Stevie - always a vocalist but his harmonica was his thing at first
Marvin - drums
Martha - secretary/back-up singer
Diane, Mary & Flo - Hand claps
Four Tops - back-up singers
Woody Plank: "Where Did Our Love Go" was my big break
Annie Lennox-flute
Elton John... wasn't he a session pianist?
Ray Parker Jr.
Melissa Manchester
Karen Carpenter
Lorraine Hunt (Lieberson) started out as a last stand violist in the Boston Ballet orchestra.
Gee, R17. What about Nat "King" Cole, Louis Armstrong and George Benson? You're either lazy or a racist.
Annie Lennox-flute
she at the guy, David, whatever, went to RCM
Herb Alpert?
Herb Alpert sand a few times, but he was hardly a singer.
Stevie Ray Vaughan
Philippe Jaroussky
Jimi Hendrix
Madonna - drummer in "The Breakfast Club"
Glen Campbell, legendary session guitarist.
Barry M.
Divine Miss M.
Vince Gill
Mel Torme (drummer in Chico Marx's band)
DL fave Kaye Ballard (flautist with Spike Jones)
Robert Cray. Was he though? Cray?
Sorry.
Barry Manilow started out as a pianist, including a stint as Bette Midler's accomanist.
I heard him interviewed on NPR's "Fresh Air", and he was oddly proud of his years as an accompanist, like it was the only real job he'd ever held.