James cleared waivers after the Mavericks were forced to turn him loose for salary cap reasons. Ellington agreed to sign with Dallas in early July after free agency opened.
Terms of the deals announced Friday weren't disclosed.
James, a former Air Force staff sergeant, was the 33rd pick in the 2012 draft. The Mavericks acquired the rights for James, Jared Cunningham and Jae Crowder in a deal involving No. 17 pick Tyler Zeller.
James averaged 2.8 points and 2.8 rebounds in 46 games, including 11 starts.
Ellington is a four-year veteran with career averages of 6.9 points and 2.0 rebounds. He has played for Minnesota, Memphis and Cleveland.