"It is with deep and profound sadness that the family of former Secretary of Defense Ashton B. Carter shares that Secretary Carter passed away Monday evening in Boston after a sudden cardiac event," his family said in a statement.

"He was a beloved husband, father, mentor, and friend. His sudden loss will be felt by all who knew him."

Carter held the top job at the Pentagon from 2015 to 2017 -- a period that covered the height of the war against the Islamic State (IS) group, which lost swathes of territory it seized in Iraq and Syria to local ground forces backed by a US-led international coalition.

He also oversaw sweeping changes to the US military, including the opening of combat positions to women and the lifting of a ban on transgender personnel openly serving in the armed forces.

Donald Trump sought to reverse the latter move during his presidency, but his administration's restrictions on transgender personnel were overturned by Joe Biden when he took office in 2021.

Biden -- who was vice president during Carter's time as defense chief -- praised him as a "great American of the utmost integrity."

He "was a leader on all the major national security issues of our times -- from nuclear deterrence to proliferation prevention to missile defense to emerging technology challenges to the fight against Al-Qaeda and ISIS," Biden said, using an acronym for IS.

Obama also mourned his former defense secretary, saying he was "a keen student of history, a brilliant physicist, and a steadfast defender of our men and women in uniform."

He "left America -- and the world -- safer through his lifetime of service," the former president said.

And US Defense Secretary Lloyd Austin said Carter "was both a defense intellectual and a skillful policymaker who tirelessly sought a more secure America in a more just world."

After leaving the Pentagon, Carter became the director of the Belfer Center for Science and International Affairs at Harvard's Kennedy School.

He is survived by his wife and two children.