Ted Kennedy died in 2009 - nine years to the day that McCain died, and of the same form of brain cancer.
In addition to signing the proclamation, Trump announced that he'd asked Vice President Mike Pence to give an address at the ceremony honoring McCain at the Capitol on Friday.
As Nicolle Wallace covered today's newest developments on her MSNBC show, a Washington Post White House reporter noted Trump had ignored shouted questions about McCain earlier today, adding that Trump is clearly a man who gets "swayed by news coverage, who watches cable news".
White House chief of staff John Kelly, Defense Secretary James Mattis and National Security Advisor John Bolton will represent the administration during services. Typical practice is for flags to remain at half-staff through the day of interment.
At the request of the McCain family, I have also authorized military transportation of Senator McCain's remains from Arizona to Washington, D.C., military pallbearers and band support, and a horse and caisson transport during the service at the United States Naval Academy.
After Democratic Ted Kennedy of MA died in 2009, U.S. President Barack Obama ordered flags at the White House flown at half-staff for five days.
The White House flag was eventually re-lowered, and shortly thereafter President Trump issued his first non-Twitter statement - almost 44 hours after McCain's death was reported.
"Mr. President, just this year, you released presidential proclamations noting the deaths of Barbara Bush and Billy Graham".
Wallace then said, "I did watch some Fox News on Saturday night, and even his favorite network Fox News celebrated American hero John McCain".
His family shared Friday that McCain wouldn't continue with treatment for his brain cancer, and the senator died the next day. "At his death, he had served the United States of America faithfully for sixty years".
"I doubt you could find a comparable situation where the president doesn't order the flag flown at half-mast until the funeral", said John Lawrence, history professor at the University of California's Washington Center.
Canadian Prime Minister Justin Trudeau called McCain "an American patriot and hero whose sacrifices for his country, and lifetime of public service, were an inspiration to millions". Vice President Joe Biden is also slated to attend and deliver remarks at the ceremony.
McCain will lie in state in the Arizona Capitol on Wednesday, the day McCain would have turned 82 years old, according to the state's governor, Doug Ducey.