Roark will lead field and newsroom and newsgathering teams as well as CBS News Radio.
She will oversee 23 of the 27 CBS-owned stations.
Mitchell had been president of CBS Stations (West Coast and Midwest).
Also announced were new roles for other news executives.
He will report to McMahon.
She also will continue to oversee weather coverage for the division.
Stewart and Fisher will report to Roark.
David Reiter has been named senior vice president, CBS News 24/7, special events and special projects.
The role includes responsibilities for special coverage on television and leading the divisions streaming initiatives.
He will report to Roark.
Margaret Brennan will be a regular presence on the evening news, leading political coverage from Washington, D.C.
The changes also come as CBS parent Paramount Global unveiled plans to trim $500 million in costs.
That was followed by the announcement of a planned merger of Paramount Global with Skydance Media.
