The Supreme Court Monday morning dismissed a pair of cases alleging that former President Donald Trump inappropriately profited off the presidency in violation of the Constitution's emoluments clause.

The move brings to an end a four-year challenge by Maryland, Washington, D.C., and a group of restaurants and hotels that alleged Trump's hotel in the nation's capital became a hub of foreign patronage and unfair competitive advantage.

The cases put a spotlight on emoluments and gave the otherwise arcane constitutional provision a starring role in the Trump presidency.

The Supreme Court declared both cases "moot" now that Trump has left office, but did not elaborate further on the decision. It did not provide a breakdown of how the justices may have voted.

-ABC News Senior Washington Reporter Devin Dwyer

Incoming Defense Secretary Lloyd Austin -- the Defense Department's first Black chief -- was ceremonially sworn in by Vice President Kamala Harris -- the nation's first Black vice president -- in the Roosevelt Rom of the White House on Monday afternoon.

Austin's wife, Carlene, held the Bible her husband placed his hand upon for the ceremony.

Moments earlier, Austin stood alongside Biden in the Oval Office as the president signed an executive order stating all Americans, regardless of gender identity, can serve in the Armed Forces.

“I fully support the President’s direction that all transgender individuals who wish to serve in the United States military and can meet the appropriate standards shall be able to do so openly and free from discrimination," Austin said in a statement Monday.

-ABC News' Matthew Seyler

Sen. Patrick Leahy, D-Vt. -- the Senate president pro tempore -- is expected to preside over the impeachment trial of former President Donald Trump.

Senators preside when the impeached person is not the president of the United States, according to a Senate source.

The trial is slated to begin the week of Feb. 8 following House impeachment managers delivering the article of impeachment for "incitement of insurrection" to the Senate Monday evening.

Supreme Court Chief Justice John Roberts presided over Trump's last impeachment trial.

-ABC News' Trish Turner

Biden has officially reversed a ban on transgender people from serving in the U.S. military, signing an executive order in a brief ceremony in the Oval Office with Defense Secretary Lloyd Austin and Joint Chiefs Chairman Mark Milly looking on.

"What I'm doing is enabling all qualified Americans to serve their country in uniform," he said.

The White House had announced earlier he had done so in a release.

The controversial ban was put in place by former President Donald Trump in 2017 and reversed the Obama administration’s policy to allow open service by transgender people.

-ABC News' John Verhovek

Clarification: This story has been updated to reflect that the White House press release was issued before the actual signing.