WATCH: Trump biographer reveals how he knows the president 'makes up' figures about his wealth

A biographer who's been investigating Donald Trump for decades revealed to MSNBC's Ari Melber how he knows the president lies about his wealth.

"Back in 1999, Donald Trump told me he was worth $3 billion," biographer David Cay Johnston said. "I said, 'Donald, I don't believe you.'"

At the time, Trump was unable to pay his bills and "all sorts of people" weren't being paid, Johnston noted.

"If you're a billionaire, you would sell an asset, do something, to pay these people," the biographer noted. "That very day, he told another reporter he was worth $5 billion. He just makes it up."

The "best proof" of that assertion? When Trump was a candidate, Johnston said, he claimed to be worth more than $10 billion — but once he was sworn in as president, he had to sign a statement under oath that said his net worth was only $1.4 billion.

"$9 billion, poof," the biographer said. "He says it was never there."

Melber noted that even the New York Times, whose bombshell report about Trump family tax fraud made waves even amid the circus surrounding Brett Kavanaugh's impending confirmation to the Supreme Court, admitted that the newspaper played into the "myth" of Trump as a billionaire.

Reporters like Johnston always called BS, the host suggested — and were once upon a time echoed by Republicans like Marco Rubio, Ted Cruz and Mitt Romney.

Melber then played clips of Trump's former primary opponents criticizing him for lying about his wealth.

Watch below, via MSNBC: