The Reds forward had hit the post in normal time, but with 99 minutes gone in Qatar he made no mistake when he rounded the goalkeeper and two covering defenders to slot home the winner.
Here are five talking points from an historic night...
Officially the best team on the planet
World and European champions! It certainly has a good ring to it, doesn't it?
Jürgen Klopp's men will be the first ever Liverpool side to add the official FIFA Champions Badge to their shirts after succeeding in the club's fourth attempt to conquer the globe.
Reds teams led by such luminaries as Bob Paisley (1981), Joe Fagan (1984) and Rafael Benitez (2005) have come up short in the past, but in Qatar, this squad delivered.
The Club World Cup is the third piece of silverware Liverpool have claimed this year, following on from the Champions League trophy and UEFA Super Cup won earlier in 2019.
That's history, too, as no English side has ever scooped those three major trophies in the same year.
And as for the manager, Klopp becomes the first German coach to lift the trophy since the competition adopted its current format in 2005.
Bobby delivers... again
Having come off the bench to win Liverpool their semi-final three days earlier, Firmino once again stepped up when his team really needed a goal in Qatar.
Played in by Sadio Mane with 99 minutes gone and the game into extra-time, the Brazilian drove into the box, side-stepped the goalkeeper and found a gap between defenders to calmly slot in the winning goal.
And in doing so, Firmino became the first Reds player ever to score in a Club World Cup final.
He had come agonisingly close to doing so on 47 minutes, but after brilliantly hooking the ball over the head of Rodrigo Caio his subsequent shot hit the post and bounced across the line and out.
Gomez stars alongside Van Dijk
Reds fans were delighted to see Virgil van Dijk's name back in the starting XI after he missed the semi-final due to illness, but his fellow centre-back Joe Gomez really caught the eye against Flamengo.
After helping talk makeshift defender Jordan Henderson through that victory over Monterrey earlier in the week, Gomez really flourished in the final.
He made a brilliant last-ditch block on Bruno Henrique on 26 minutes that set the standard defensively and was just as comfortable moving into midfield with possession during an excellent all-round performance.
At the end of the 90 minutes, he led the team in touches (102), passes (89), successful passes (79), duels (15) and tackles (five).
Alisson at it again
When Flamengo did manage to break through Liverpool's rearguard, they found goalkeeper Alisson Becker in simply unbeatable form.
Just as he had done in the semi-final against Monterrey, the No.1 made crucial saves at crucial moments.
Whether it was racing off his line to beat Gabriel Barbosa to a ball over the top or denying the striker with a strong right-handed save at full stretch, his full repertoire was on show at Khalifa International Stadium.
In a tie that ebbed and flowed, Flamengo enjoyed dominant moments in the game, but Alisson helped ensure his Brazilian compatriots, who had 12 shots over the course of the match, were not able to make those periods count.
Henderson leads by example
Another game, another outstanding performance from the captain, who shielded his back four in the No.6 role and posed a threat going forward too.
Denied by a great save from Diego Alves in the final few minutes of the 90, it was his pass to Mane that set up the winner for Firmino.
On a night when emotions occasionally threatened to spill over, the skipper also helped keep his players focused on the immediate task in hand - winning the final.
He led the team in possession regains (12), won 100 per cent of his aerial duels, and was second only to Gomez in total passes made, with 67, as he kept the Reds ticking over.
Henderson fully deserved to showcase what has become a trademark trophy lift - his third of the calendar year.