The result left both sides on five points in the mini-league, but the Lions won the tie by virtue of having scored two aggregate points more following their 26-19 first leg win in Port Elizabeth on July 26.
"It was not the result we were after, but we are back into Super 15 next season," Lions captain JC Janse van Rensburg said in a TV interview.
"We wanted to get back what belongs to the Lions. We knew we had to go down to Port Elizabeth to get points, we had a seven point buffer, we wanted to win tonight, but it was a helluva hard game."
The Kings needed to win the game by seven points to retain their Super Rugby status, but tries from Derick Minnie and Jaco Kriel, as well as two penalties and a conversion from flyhalf Elton Jantjies, who was loaned to the Stormers this season, put the Lions in control at 18-9 after 55 minutes.
A yellow card 11 minutes later for Lions centre Stokkies Hanekom gave the Kings hope and they scored two quick tries through Schalk Ferreira and Scott van Breda.
The latter, taking over the kicking duties from the injured Demetri Catrakilis, converted both to add to his earlier three penalties and suddenly the Kings were within three points of salvation.
But the home side held on for the final seven minutes in front of a raucous capacity crowd to relegate the Kings.
"I thought we had a great Super Rugby season and the guys gave it a great effort, but it was small margins again today that the Lions pipped us at," said visiting captain Darron Nell.
"Congratulations to them. We will go back on Monday, get ourselves together and look forward from there."
The Kings won plenty of friends in Super Rugby this season with some gutsy performances, but just three of their 16 matches as they finished bottom of the South African conference to go into a playoff for survival.
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