Sweeney said in a brief statement that he planned to return to the private sector, giving no reason for his departure. His exit comes just days after Secretary of Defense James Mattis, for whom Sweeney previously served, left the agency after announcing his resignation earlier in the month.
"After two years in the Pentagon, I've decided the time is right to return to the private sector. It has been an honor to serve again alongside the men and women of the Department of Defense," Sweeney said in his announcement, The Hill reported.
His resignation also followed that of another top Mattis ally, Pentagon spokeswoman Dana White, who announced her resignation at the end of December.
“I appreciate the opportunity afforded to me by this administration to serve alongside Secretary Mattis, our Service members and all the civilians who support them. It has been my honor and privilege,” White wrote on Twitter last month. “Stay safe and God bless.”
Mattis's resignation was thought to be tied to the surprise announcement from President Trump regarding the withdrawal of US troops from Syria, where coalition forces continue to battle elements of the Daesh (also known as ISIL or ISIS) in Iraq and Syria (ISIS) on the ground.
Trump's withdrawal of US forces from Syria was also credited at the time with pushing the resignation of Brett McGurk, the president's top envoy to anti-ISIS coalition forces in the region.
Mattis's former deputy, Patrick Shanahan, began serving as acting Defense secretary at the beginning of January.