Y: 1883 depicts the Dutton family heading west to Montana. The original series takes place in the present with John Dutton (Kevin Costner) and his children conflicting over the ranch.
Elliott will play Shea Brennan, a cowboy guiding a group from Texas to Montana. Paramount describes Brennan as "tough as nails" with a tragic past, who "does not suffer fools."
"The classic struggles of man against man, man against nature and man against himself, it's all there," Elliott said in a statement.
Real-life spouses Hill and McGraw will play James and Margaret Dutton, ancestors of John.
"The Duttons are a formidable family and it is an absolute dream to bring such a strong female character like Margaret Dutton to life," Hill said.
Yellowstone creator Taylor Sheridan also created the prequel. Yellowstone airs on the Paramount Network and currently streams its first three seasons on Peacock.
Elliott has a history in westerns like Tombstone and TV miniseries The Sacketts and Wild Times. Singer McGraw has appeared as an actor in movies like The Blind Side, Friday Night Lights and Country Strong.
Hill acted in movies The Stepford Wives, Dixieland and episodes of Touched By an Angel and Promised Land. Y: 1883 is her first series regular role.