The England defender, like many of his team-mates, endured mixed fortunes in the previous campaign as he looked woefully short of form under Roy Hodgson but was rejuvenated after Kenny Dalglish's arrival in January, playing well out of position at left-back.
Johnson committed his long-term future to the club on Wednesday and admitted it was an easy decision.
"I'm very pleased. I've been here a couple of years and it's great to be staying here for a few years longer now," he said.
"For the club to want me to be here is a great feeling and I did not have to think twice about signing.
"I think I am improving all the time at Liverpool. You're never too good to improve - and my aim is to just keep improving.
"With the squad we've got now, the backroom staff and the way things are going, things are looking positive. The club is looking to bring a few more players in, so the future is looking bright.
"We've made a couple of good signings already and with a few more we will be a right force in the coming season.
"I'm quietly confident (about the new season). If we take it game-by-game I think we'll do well."