An emphatic 4-2 win at Bristol City, who are second in the Women's Championship ensured Liverpool went back up to the WSL with two games to spare.
Liverpool captain Niamh Fahey gave the away side an early lead at Ashton Gate, before Aggie Beever-Jones got the hosts back on terms a minute later.
Jasmine Matthews put Liverpool 2-1 up moments before before half-time, with Katie Stengel adding a third just before the hour mark.
Bristol City threatened a comeback through Abi Harrison's 15th goal of the season, but Liverpudlian Missy Bo Kearns ended the tie with a fourth goal.
It's been a fantastic campaign from Matt Beard's side, who have won 15 from 20 games, losing just once. They've scored 42 times and conceded just eight.
Now after a two-year absence, Liverpool, who were relegated in the 2019/20 season, are back in the big time.
And James Milner as well as Andy Robertson were keen to applaud their efforts.
On Twitter, left-back Robertson wrote: "Love it".
And Milner said on Instagram: "Congratulations to @LiverpoolFCW on winning the @FAWomensChampionship and getting promoted back in to @FAWSL."
Meanwhile, Alexander-Arnold said in a video message: "Hi everyone, just wanted to send a quick message to say congratulations to everyone involved with LFC Women.
"Amazing achievement, you've done so well putting in all the hard work this season and it's amazing to see. So big congratulations to Matt and all the staff and all the players. A big congratulations, well done!"
The Reds will want to keep up their form though and finish the season as strong as possible.
They finish of their home campaign on April 24 against fourth placed Sheffield United.
The season then comes to an end on May 1 which sees Liverpool take on Lewes.