Paul Gallacher marked his 50th Football League start with a superb 30-yard strike to give Leicester the lead after 15 minutes.
Wales international midfielder King made it 2-0 two minutes before half-time but Jacob Mellis scored in first-half injury time to give Barnsley hope going into the break.
King restored the two-goal cushion just before the hour mark before Kyle Naughton completed the scoring with another long-range cracker in the final minute.
It secured a fifth win in six league matches for Sven-Goran Eriksson`s side, who are up to eighth in the table and three points shy of the play-off positions.
Source: PA
Source: PA