Working for Marvel, Thomas was the first person to bring Robert E. Howard's legendary pulp hero to the comics medium in 1970.
Thomas will write the 12-issue Conan: Road to Kings limited series alongside artist Mike Hawthorne.
The series will launch after the conclusion of Dark Horse's Conan the Cimmerian ongoing title, which ends at issue #25.
"I'd always sort of wanted to write Conan, but [Dark Horse] felt they wanted to go with new art, writing, kind of a new direction, establish their own identity," Thomas told CBR.
"And then recently, Mike [Richardson, Dark Horse publisher] and I began to talk about the possibility of me doing something with one of the Robert E. Howard characters, one that they hadn't done a series for, and I made a few suggestions.
"They sounded okay to Mike, but then suddenly he and some of his editors came up with this fact that in 1970, 40 years ago this summer, was the debut of Conan the Barbarian #1. We decided to do a series that'll run about a year and begin as a 40th anniversary thing."