The 27-year-old arrived at Scotstoun in the summer of 2015 and has become a key figure for both club and country.
Having made his debut against the Ospreys in November 2015, Johnson has gone on to make 67 appearances for the Warriors to date in a spell that includes two Heineken Champions Cup quarter-final appearances and a run to the 2019 Guinness PRO14 Final at Celtic Park.
His form in a Glasgow jersey saw him earn an international debut in the 2019 Guinness Six Nations, scoring an iconic Scotland try in the thrilling 38-38 draw with England at Twickenham.
Johnson has made 15 appearances in a Scotland jersey to date, including selection for the 2019 Rugby World Cup in Japan.
Speaking to glasgowwarriors.org about the new deal, the centre said: "I'm excited to re-sign with the club.
"Glasgow is my home and to be able to sign for a few more years is a great boost for me.
"There are exciting times ahead for the club and I'm very fortunate for Glasgow and Scottish Rugby to give me the opportunity to continue to work hard with Danny and the rest of the coaching team to build something special at Scotstoun."
Glasgow Warriors Head Coach Danny Wilson welcomed the announcement, which sees another Scotland international pledge their future to the club.
"Sam's a really exciting midfield player both sides of the ball. He's an intelligent player who has a real understanding of the game, and he uses that to bring the best out of the players around him.
"He's is someone I've got to know well over the last couple of years while I've been in Scotland and I've really enjoyed working with him.
"I think it's great that we can retain another Scottish internationalist of Sam's calibre and we look forward to him pulling on the Glasgow jersey on many more occasions over the next few years."