On December 8, 2018, President Donald Trump announced that he would nominate Milley to serve as Chairman of the Joint Chiefs of Staff, although the Secretary of Defense at that time James Mattis and the incumbent Joint Chiefs of Staff Chairman General Joseph Dunford favored Air Force Chief of Staff General David L. Goldfein to be the next Chairman of The Joint Chiefs of Staff.
With Senate confirmation (89–1) on July 25 he was sworn in on September 30, 2019.

After Milley was nominated, he headed a commission with other American military officials that were responsible for designing a report on the country's impending near-term impacts from climate change.
The report, which was released in August 2019, stated that the country and its military would experience a total collapse within the next two decades due to collapses in the country's aging power grid and food supply, as well as the increased risk of infectious disease outbreaks globally.
The report also mentions the likelihood of increasing water scarcity in developing countries, which would result in an increase of civil and military conflicts due to a failure in the global food system.

After attending 75th anniversary commemorations of the Battle of the Bulge in Belgium and Luxembourg on December 16, 2019, Milley met with the Russian military chief of staff Valery Gerasimov in Bern, Switzerland, on December 18.
This continued a series of regular meetings between the American and Russian military chiefs reestablished by Milley's predecessor Joseph Dunford in 2017 to ensure open communication and avoid conflict, especially in Syria.
The face-to-face meeting was arranged with the assistance of the incoming Swiss Chief of the Armed Forces Korpskommandant (Lieutenant General) Thomas Süssli.

On June 1, 2020, during protests in Washington, D.C., in the wake of the killing of George Floyd, Milley joined National Guardsmen and various police forces assembling in Lafayette Square, across the street from the White House.
Minutes later, the troops and police used tear gas and other riot control tactics to disperse peaceable protestors so Trump could stage a photo-op at nearby St. John's Episcopal Church.
About half an hour after that, Milley, in combat uniform, walked alongside the president from the White House to the church, drawing sharp criticism from former military officers and others.
Milley subsequently refused to testify in front of Congress regarding the military's role in the response to the protests.
He reportedly considered resigning over the incident, but chose instead to address it in a video recorded as his commencement address to the National Defense University.
In that speech, streamed on June 11, Milley said he should not have been at the event because his presence created a perception of military involvement in domestic politics.
On December 8, 2018, President Donald Trump announced that he would nominate Milley to serve as Chairman of the Joint Chiefs of Staff, although the Secretary of Defense at that time James Mattis and the incumbent Joint Chiefs of Staff Chairman General Joseph Dunford favored Air Force Chief of Staff General David L. Goldfein to be the next Chairman of The Joint Chiefs of Staff. With Senate confirmation (89–1) on July 25 he was sworn in on September 30, 2019.
After Milley was nominated, he headed a commission with other American military officials that were responsible for designing a report on the country's impending near-term impacts from climate change. The report, which was released in August 2019, stated that the country and its military would experience a total collapse within the next two decades due to collapses in the country's aging power grid and food supply, as well as the increased risk of infectious disease outbreaks globally. The report also mentions the likelihood of increasing water scarcity in developing countries, which would result in an increase of civil and military conflicts due to a failure in the global food system.
After attending 75th anniversary commemorations of the Battle of the Bulge in Belgium and Luxembourg on December 16, 2019, Milley met with the Russian military chief of staff Valery Gerasimov in Bern, Switzerland, on December 18. This continued a series of regular meetings between the American and Russian military chiefs reestablished by Milley's predecessor Joseph Dunford in 2017 to ensure open communication and avoid conflict, especially in Syria. The face-to-face meeting was arranged with the assistance of the incoming Swiss Chief of the Armed Forces Korpskommandant (Lieutenant General) Thomas Süssli.
On June 1, 2020, during protests in Washington, D.C., in the wake of the killing of George Floyd, Milley joined National Guardsmen and various police forces assembling in Lafayette Square, across the street from the White House. Minutes later, the troops and police used tear gas and other riot control tactics to disperse peaceable protestors so Trump could stage a photo-op at nearby St. John's Episcopal Church. About half an hour after that, Milley, in combat uniform, walked alongside the president from the White House to the church, drawing sharp criticism from former military officers and others. Milley subsequently refused to testify in front of Congress regarding the military's role in the response to the protests. He reportedly considered resigning over the incident, but chose instead to address it in a video recorded as his commencement address to the National Defense University. In that speech, streamed on June 11, Milley said he should not have been at the event because his presence created a perception of military involvement in domestic politics.
Mark Milley