Mitchell, 33, played with the Vancouver Canucks the last three seasons. He was limited to 48 games in 2009-10 and missed the remainder of the season after suffering a concussion during a Jan. 16 game against the Pittsburgh Penguins. Mitchell had four goals and 12 points.
The 12-year NHL veteran was offered a one-year deal, heavy with bonuses, by the Canucks, according to media reports.
Mitchell played in all 82 games in 2008-09, finishing with three goals and 23 points.
Kings President Dean Lombardi said that "there is risk with any player you sign," but was satisfied that Mitchell was healthy.
"[Mitchell] passed the league requirements and having talked to our own doctors," Lombardi said. "Then he did an impressive workout on his own for us. We didn't even ask him to do it."
This was the third concussion of Mitchell's career, and by far the most severe. Mitchell said he would not entertain offers from teams until he was sure he could play.
Mitchell said he has been symptom free since mid-June and began rigorous workouts in mid-July.
"I wanted to go about things the right way and gradually build things up," Mitchell said. "I didn't want to put a general manager or team in a position they don't want to be in."