He appeared to be fine at the start, clearly mouthing along to "say can you see, by the dawn's early light...proudly we hailed".
But then his mouth appeared to go awry. He uttered something at "gleaming" and appears to correctly mouth "stripes and bright stars".
Otherwise, he either kept his mouth shut or appears to utter gibberish.
President 45 also appeared to give a quick side eye to the camera trained directly on him and at another point pats his chest while remaining silent.
The former businessman got back on track with "red glare, The bombs bursting in air".
He then sang along to "gave proof through the night" before appearing to veer off again.
On "wave", when the Anthem crescendos before its last line and people generally start clapping, Trump gives an awkward smile.
He then states: "For the land of the free, and the home of the brave" - but appears to preemptively say "brave".
Twitter was quick to take Trump - who has been a vocal critic of football players who do not stand for the National Anthem - to task for appearing to mumble his way through it.
Kasey Dixon wrote: "Does @realDonaldTrump even know the words of the Star Spangled Banner? Sure doesn't look like it..."
Puppits said: "Thought Trump looked ok until the camera close-up showed he clearly didn't know the words to the Star Spangled Banner."
Drew Gibson observed: "When the camera panned to [Trump] he just had that mouth agape, thousand-yard stare going."
Inderia Salter wistfully asked: "Where is Obama?"
But tweeters also wondered whether Trump's botched Anthem singalong was more a sign of forgetfulness and early dementia.
Robert Murphy observed: "I do not think that Pres Trump not knowing the words to the #NationalAnthem is a sign that he is unpatriotic. I think it is a sign that he has dementia, and has forgotten the words to a song that he knew most of his life."
Jeremy Newberger wondered: "Which is worse, Trump doesn't know the words to the National Anthem or Trump has dementia and can't remember the words to the National Anthem?"
Sarah Smile lamented: "Trump doesn't know the words to the national anthem- sad, so sad. Dementia is no joke. #CFPNationalChampionship #realDonaldTrump."
The White House slapped down speculation that the president is "unfit" for office last week after anecdotes in Wolff's bombshell book about the first year of Donald Trump's presidency raised new questions about the 71-year-old's mental competency.
"It's disgraceful and laughable. If he was unfit he probably wouldn't be sitting there, wouldn't have defeated the most qualified group of candidates the Republican Party has ever seen," press secretary Sarah Huckabee Sanders said.
A news report also revealed today that congressional Democrats invited a Yale psychiatrist to provide them with an assessment of president's cognitive state.
Author Michael Wolff claimed in a Thursday essay in the Hollywood Reporter that Trump is increasingly forgetful, repeating stories word-for-word inside 10 minutes and failing to recognize old friends.
The essay is based on his recently released explosive book, Fire and Fury, and includes other material.
To hide his failings from the public, Wolff claims the White House hatched a plan last fall to have interviewers submit their questions beforehand.
Trump's father, Fred, developed Alzheimer's in his 80s.
The fumbled Anthem came as Trump was jeered and booed by several protest groups that packed into an already crowded Atlanta, where the game is being held.
The Atlanta branch of the NAACP urged people to wear white to the game and to bring white towels to wave when the president enters the stadium.
The white is meant to mock the 'snowflake' insult that Trump supporters have made against those who oppose the president.
"We're going to make a snowflake turn into a mighty blizzard inside of Mercedes-Benz stadium when Mr. Trump comes," Gerald Griggs, a vice president of the Atlanta NAACP, said at a news conference Monday.
The organisation also planned a "Twitter storm" beginning at 6pm and continuing through to the end of the game.
The plan is to use the hashtag #AllTrumpsLies to highlight what they say are lies told by the president.
Another group, Refuse Fascism ATL, also demonstrated outside CNN's world headquarters near Mercedes-Benz Stadium, where the game was set to kick off at 8pm. The group planned to "take a knee" there at 6.30pm.
Atlanta police stated earlier that they set up several designated areas for protesters and won't interfere with demonstrations unless protesters break the law.
Local, state and federal law enforcement authorities said last week that they've worked for months to develop security plans.
Earlier on Monday, Trump urged respect for the American flag and the national anthem at a farmers' convention on Monday.
Trump was there to deliver a partisan message about tax cuts and regulation-slashing to America's largest agriculture group.
However, he began talking about the "great American flag" midway through.
"We are fighting for our flag. We want our flag respected. We want our flag respected,' he said twice. 'And we want our national anthem respected, also," Trump added to whoops.
Kendrick Lamar, the game's half-time performer, could also take the opportunity to rip the president after saying in October that Trump's election was a "mindf***" and it is "building the fire" for him.
A track Lamar released earlier this year, The Heart Part 4, hits Trump as a "chump" and claims that Russia is "up to something".
Trump said Monday afternoon in Tennessee that there's "plenty of space for people to express their views and to protest.
"But love our flag, we love our anthem, and we want to keep it that way," the president stated.
At a rally in Alabama last year, Trump issued a sharply worded warning to players who do not stand for the National Anthem.
The move was inaugurated by ex-49ers Colin Kaepernick during the 2016-2017 season as a means to protest police violence against African Americans.
"Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say, 'Get that son of a b***h off the field right now! He is fired. He's fired!'" Trump boomed at the rally held in September 2017.
His crowd applauded and chanted "USA! USA!"
