Before pardoning Dinesh D'Souza -- a caustic critic of Barack Obama and Hillary Clinton who was convicted of campaign finance violations -- Trump tweeted that the author and filmmaker had been treated "very unfairly" by the government.
The US leader later said he was also considering commuting the sentence of Rod Blagojevich, a former Illinois governor jailed for corruption, and Stewart, the celebrity chef jailed for five months in 2004-05 for making false statements in an insider trading case.
Stewart once hosted a spin-off version of Trump's reality TV show "The Apprentice" while Trump noted that Blagojevich -- a Democrat -- had appeared on the celebrity version of the show.
Speaking to reporters on Air Force One, Trump complained that Stewart had been "harshly and unfairly treated."
"And she used to be my biggest fan in the world... before I became a politician," he said.
The latest figure to receive a pardon, the Republican pundit and provocateur D'Souza, was fined $30,000 and sentenced in September 2014 to five years of probation for making illegal campaign contributions.
In a statement, the White House said the 57-year-old was "in the President's opinion, a victim of selective prosecution for violations of campaign finance laws.
"Mr. D'Souza accepted responsibility for his actions," it said. "In light of these facts, the President has determined that Mr. D'Souza is fully worthy of this pardon."
D'Souza pleaded guilty to attempting to donate $20,000 to a Senate campaign through straw donors.
He also personally contributed $10,000 to the campaign of Republican Wendy Long, who ran unsuccessfully for Senate in New York in 2012 -- twice the legal limit for individual contributions at the time.
D'Souza took to Twitter to thank Trump.
"Obama & his stooges tried to extinguish my American dream & destroy my faith in America," he said. "Thank you @realDonaldTrump for fully restoring both."
D'Souza's 2010 book "The Roots of Obama's Rage" was made into a film, "2016: Obama's America," which was popular with conservative groups.
He also made a documentary about Clinton -- "Hillary's America: The Secret History of the Democratic Party."
'Voluntarily pled guilty'
Regarding Blagojevich, Trump said he was considering commuting his sentence, which he described as "really unfair" punishment for "foolish" behavior.
"I'm seriously thinking about it," Trump said.
Blagojevich was impeached as governor of Illinois in 2009 and sentenced in December 2011 to 14 years in prison for corruption including trying to sell the Illinois Senate seat vacated by Barack Obama.
"Plenty of other politicians have said a lot worse," Trump said. "He shouldn't have been put in jail."
Blagojevich was fired by Trump in episode four of the 2010 season of "The Celebrity Apprentice."
D'Souza claimed before pleading guilty that he had been targeted for prosecution because of his anti-Obama views.
Judge Richard Berman dismissed the bias claims. "The defendant's claim of selective prosecution, legally speaking, is all hat, no cattle," he said.
Former US attorney Preet Bharara defended D'Souza's conviction on Thursday.
"The President has the right to pardon but the facts are these," Bharara said. "D'Souza intentionally broke the law, voluntarily pled guilty, apologized for his conduct & the judge found no unfairness."
Sending a message?
Trump has granted five pardons since taking office including to Joe Arpaio, a Trump supporter who was convicted of contempt of court for racially profiling Hispanics while serving as a sheriff in Arizona.
Trump also pardoned Lewis "Scooter" Libby, former vice president Dick Cheney's chief-of-staff who was convicted of obstruction of justice and perjury, and Kristian Saucier, a US Navy sailor convicted of taking photos inside a nuclear submarine without permission.
Last week, Trump issued a posthumous pardon to Jack Johnson, the first black heavyweight boxing champion, who was sent to prison a century ago in a racially charged case.
According to some political analysts, the pardons could be a signal to Trump allies who are currently facing charges or are under investigation.
"This could be a message to Paul Manafort," CNN legal analyst Jeffrey Toobin said of Trump's former campaign manager who has been indicted by special counsel Robert Mueller for money laundering and tax evasion.
"This could be a message to Michael Cohen," Toobin said of Trump's personal attorney who is under criminal investigation for unspecified crimes.
Trump's message could be: "Hang in there the cavalry is coming," Toobin said.
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