Now he's preparing to mark Cristiano Ronaldo in next week's Champions League final and is the emblematic figure of a youthful and boldly assembled England squad heading to the World Cup in Russia.
Alexander-Arnold was the headline pick Wednesday in a 23-man group containing 449 caps, making it the most inexperienced World Cup squad selected by England this century. Only two players are over the age of 30, and only one player has more than 38 caps.
"I believe this is a squad which we can be excited about," said England coach Gareth Southgate, who stuck to his footballing beliefs and didn't fall into the trap that some of his predecessors have of opting for established names over form players.
Long-time goalkeeper Joe Hart and injury-prone midfielder Jack Wilshere were omitted, for example. Chris Smalling, who has had a good season for Manchester United, was also overlooked because Southgate wants defenders who are comfortable in possession.
Southgate has been strong-willed in his 18 months in charge — he virtually brought Wayne Rooney's long England career to a close — and he hasn't buckled under pressure.
"We have a lot of energy and athleticism in the team," Southgate said, "but players that are equally comfortable in possession of the ball and I think people can see the style of play we've been looking to develop."
Alexander-Arnold has all that, and more. He only made his senior debut for Liverpool only 17 months ago but is now a regular at right back in Juergen Klopp's team, which will take on Real Madrid in the Champions League final on May 26.
Lean, strong in the tackle and athletic, Alexander-Arnold is the archetypal modern-day full back — attacking, energetic, and with a good delivery. He often takes free kicks and has played in midfield in recent Premier League matches.
The teenager is uncapped, although he was invited to train with England in March.
"Dreamt of going to a World Cup since I was a kid," tweeted Alexander-Arnold, who — to some — is still is a kid. "Today that dream came true."
Around the same time, another youngster, 21-year-old Man United striker Marcus Rashford, was thanking his mother in a tweet for "years of you standing on the touch line in the cold and rain."
"Mum we're off to the World Cup!" Rashford said.
Ruben Loftus-Cheek is another bold selection by Southgate, a midfielder who wasn't deemed good enough to play for Chelsea this season but has impressed on loan at Crystal Palace and starred on his international debut against Germany in November.
The inclusion of versatile players like Alexander-Arnold, Ashley Young and Fabian Delph, who can all play in defense and midfield, provides options for Southgate, who has tinkered with 4-3-3, 3-4-3 and 3-5-2 formations.
Central midfield seems light on experience and cover — Jordan Henderson and Eric Dier are the only real mainstays — but Southgate is confident the likes of Loftus-Cheek and Jesse Lingard can fill in.
Hart was England's first-choice goalkeeper at the last World Cup and the two most recent European Championships, and was Southgate's No. 1 only five months ago.
However, he has fallen behind Jordan Pickford, Jack Butland and Nick Pope after making errors and losing his place during a loan spell at West Ham from Manchester City.
Wilshere last played for England in one of the country's most humiliating matches, the 2-1 loss to Iceland in the last 16 of Euro 2016.
England is in Group G with Belgium, Panama and Tunisia.
Goalkeepers: Jack Butland (Stoke), Jordan Pickford (Everton), Nick Pope (Burnley)
Defenders: Trent Alexander-Arnold (Liverpool), Kieran Trippier (Tottenham), Danny Rose (Tottenham), Ashley Young (Manchester United), Fabian Delph (Manchester City), Kyle Walker (Manchester City), John Stones (Manchester City), Harry Maguire (Leicester), Gary Cahill (Chelsea), Phil Jones (Manchester United)
Midfielders: Eric Dier (Tottenham), Jesse Lingard (Manchester United), Ruben Loftus-Cheek (Chelsea), Dele Alli (Tottenham), Jordan Henderson (Liverpool)
Forwards: Harry Kane (Tottenham), Jamie Vardy (Leicester), Marcus Rashford (Manchester United), Raheem Sterling (Manchester City), Danny Welbeck (Arsenal)
Steve Douglas is at www.twitter.com.sdouglas80Copyright 2018 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.