Former defense secretary Jim Mattis plans to remain a prominent voice on national security issues in his post-military career as a counselor to the business advisory group led by former defense secretary William Cohen.

In a release Sunday, The Cohen Group announced that Mattis would be a senior counselor for the firm effective Oct. 1.

Since his resignation in December following a policy dispute with President Donald Trump, Mattis has also joined the board at General Dynamics and resumed his role at Stanford University as Davies Family Distinguished Fellow at the Hoover Institution at Stanford.

"We are honored to be working with [Mattis], and our clients will benefit from his knowledge and guidance," said Cohen, who served as defense secretary from 1997 to 2001 and was previously a Republican senator from Maine. "Jim Mattis is a national treasure, a leader known throughout the world for his strategic judgment as well as his unyielding principles."

A spokesman for The Cohen Group declined comment on what Mattis will be paid.

General Dynamics also did not disclose any information on Mattis' compensation when he rejoined the board of directors. Mattis previously served on General Dynamics' board from 2013-17 before becoming SecDef.

According to the organization, "The Cohen Group provides its clients the insights needed to better understand and shape the business, political, legal, regulatory, and media environments in which they operate."

Other senior leaders at the firm include retired Air Force Gen. Joseph Ralston, former NATO Supreme Allied Commander and vice chairman of the Joint Chiefs of Staff; Nicholas Burns, former under secretary of State for Political Affairs and U.S. Ambassador to NATO; and retired Admiral Jim Loy, former commandant of the Coast Guard and a former acting secretary of Homeland Security.

-- Richard Sisk can be reached at Richard.Sisk@Military.com.

Read More: Pentagon Suspends Mental Health Counseling Referral Services for DoD Civilians