The Scotland international was just over 18 months into his original contract, signed when he arrived for a bargain £8million from Hull in the summer of 2017.
After a slow start to his Liverpool career, with manager Jurgen Klopp giving the 24-year-old time to adapt to a new way of working, the left-back is now one of the first names on the team sheet after a string of impressive performances.
Robertson is the latest in a growing list of first-teamers to commit their long-term futures to the club after Mohamed Salah, Sadio Mane, Roberto Firmino, Sadio Mane and Joe Gomez all extended their stays in the last six months.
“As soon as the club came to me, it was a no-brainer for me - I want to stay here, so as soon as they put an offer on the table it was signed as quickly as that,” Robertson told liverpoolfc.com.
“It was a pretty easy contract for me and I’m sure for the club as well. We both agreed very quickly, that’s why it’s been done so quickly.
“I am glad I have extended my stay and hopefully we have a lot of good days ahead.”