The 19-year-old signed a new deal until 2023 ahead of his move to the West Lothian side.
Williamson was hugely impressive on loan at Arbroath in the Championship last season, catching the eye across an 18-game spell.
His performances landed him the Smokies’ young player of the year award plus a well deserved call-up to the Scotland Under-21 squad.
“I watched Ben action a number of times last season whilst watching Jack Hamilton and Carlo Pignatiello at Arbroath and was always impressed”, Livingston boss David Martindale told the club's website.
"I really believe he has a great future in the game and I’m happy to get him on board here.
“Ben hasn’t long made his Scotland U21 debut which tells you the quality the lad possesses.
"I must say a big thank you to Rangers Football Club, Ben and his agent Pete Cormack for working with us to make the deal happen.”