On the heels of a scoreless draw with Manchester United, the Reds exploded for five goals in the 5-0 victory over Watford to maintain their spot atop the Premier League.
Alexander-Arnold assisted for three of those five goals, setting up Sadio Mane twice early on as well as Virgil van Dijk from a second-half free kick.
The full-back became the youngest player to assist three goals in a single Premier League match with his trio of assists coming at 20 years,143 days.
With Wednesday's efforts, Alexander-Arnold doubled his assist tally to six for the season in 18 appearances.
His first contribution came in the ninth minute as the full-back whipped in a ball from the right side that found Mane for a headed finish.
The second was more to Mane's credit, as the winger bounced back from a heavy touch to fire a magnificent backheel finish past Ben Foster.
With the two goals, Mane has now had a hand in eight goals in just five Premier League matches against Watford since moving to Liverpool.
The goals also bring him up to 14 Premier League finishes for the season, to mark his best-ever haul in the competition.
Divock Origi joined the fray with a goal of his own in the 66th minute, pushing the lead to 3-0 while all but ending Watford's hopes.
Van Dijk's pair of finishes just padded the scoreline as the Netherlands international scored in the 79th and 82nd minutes to complete the lopsided scoreline.
The loss also extends Watford's misfortunes against the Reds, as the club have lost 11 of their 12 top-flight away games against Liverpool.
The only exception was a 1-0 victory in August 1999 in what was the club's first ever Premier League game at Anfield.
Watford, sitting in eighth place, will take on newly-appointed Brendan Rodgers as the former Liverpool boss takes charge of Leicester City on Sunday.