The deal was announced by general manager Dean Lombardi following Los Angeles' 6-4 victory over the Columbus Blue Jackets on Saturday night.
The deal with the fifth-year defenceman was completed on Friday. Johnson signed the paperwork after Saturday's morning skate, ending more than two months of negotiations.
"It wasn't any kind of burden at all," said Johnson, who turns 24 on Thursday. "You just go out and play hockey every night, try to play well, and everything else would take care of itself. We had talked about numerous options and situations, and I had no problem committing that long.
"We were just trying to find something that was reasonable and fair for both sides, and I think we're all very happy about it."
Johnson, the No. 3 pick in the 2005 draft by the Carolina Hurricanes, was dealt to Los Angeles in a multiplayer deal. He has spent his entire NHL career with the Kings.
"That's great for Jack," coach Terry Murray said. "He's one of the premier young defenceman to come out of junior hockey in the last decade. So it's great to get him locked up and know that Jack's going to be a part of this franchise for a long time.
"Jack wants to become one of the top guys in the game, so this will be the place where it's going to happen," Murray added. "Young defencemen like him with skill, talent and ability like that are very hard to find. We're hoping that he and Drew Doughty can end up being the anchors of this blue line corps for a long time."
The deal matches the second longest in club history. Centre Anze Kopitar signed a seven-year contract extension in October 2008, and Wayne Gretzky signed an eight-year contract in 1988. After the first year, Gretzky's contract was extended from seven years to nine.
"Good for him. Seven years," Kopitar said. "What's most important is that I'll get a free dinner out of it I hope."