Neil Cugley’s side needed a point to be sure of reaching the play offs but both scored and won at Chipstead’s High Road for the first time in three visits.
Central defender Frankie Chappell had opened the scoring during the first half before King took his tally to 26 goals in 26 starts in his first season in Invicta colours to take him level with Jason Prior at the top of the league goalscoring charts.
Chappell has himself now completed three consecutive seasons ever present for Invicta.
The Folkestone side had Jack Delo back in goal after a one-match suspension and on a sunny afternoon the visitors had the game’s first chance when King shot wide across goal after Josh Burchlel had lifted the ball over to send him clear in a move started by captain Darren Smith.
Home appeals were then waved away in the seventh minute when striker Fabian Batchelor went down under challenge from central defender Liam Friend down the left hand side.
Batchelor shot at Invicta goalkeeper Delo from the left hand side of the penalty area four minutes later after centre half Rob Watkins had run forward.
Chipstead had one-time Invicta youngster Luke Coleman in attack but his afternoon was to end in sad circumstances as he was caught by the studs of Friend and was clearly in distress as he received treatment on the touchline. Coleman then had to be taken to hospital by ambulance at half time.
As play continued, Invicta themselves had penalty appeals waved away when a 33rd minute Liam Dickson cross from the left struck left back Jordan Brown on a hand.
A Roland Edge shot curled over the crossbar when a James Everitt ball in came back out to him.
And Invicta opened the scoring in the 38th minute when CHAPPELL poked the ball into the net after Smith and Burchell had played the ball through down the right hand side of the penalty area.
Chipstead won the game’s first corner in the 39th minute when a tom Battams shot deflected over off Friend after substitute Andy Dalhouse’s shot had been blocked.
Another Dalhouse shot flashed past the post from just outside the penalty area early in the second half.
But Invicta made it 2-0 in the 53rd minute when KING found the net as he latched onto a ball down the middle after Burchell and Darren Marsden had combined.
Burchell shot wide when he let fly from the left hand side in the 59th minute, while Batchelor ran at the Invicta defence but shot wide across goal a minute later.
DALHOUSE pulled a goal back for the home side in the 61st minute when he scored from close range after a low ball in from the right from fellow substitute Alex Wilkes in a move also involving Brown, Russell Bedford and Bruce Hogg.
Invicta restored their two-goal advantage in the 83rd minute when substitute Micheal Everitt found younger brother James and he pulled the ball back for KING to apply a trademark finish into the far corner of the net.
A stoppage time shot on the turn from Dalhouse bounced up at Delo who palmed the ball away with his hand.
But KING had the final say as he completed his second hat-trick of the season by smashing the ball into the roof of the net after receiving the ball from a Lee Gledhill throw in on the right.
It was King’s fourth goal in two games since his return from a three-match suspension.
For Invicta it proved to be an enjoyable final game of the league season before the drama of the play offs.
CHIPSTEAD: Grant, Newman, Brown (O’Shea 74 mins), York, Watkins, Rice (Wilkes 45 mins), Bedford, Battams, Coleman (Dalhouse 28 mins), Batchelor, Hogg. Unused subs: Conroy, Grizzle.
FOLKESTONE INVICTA: Delo, Gledhill, Burchell, Friend, Chappell, Dickson (M. Everitt 69 mins), Smith, Marsden (Abel 89 mins), J. Everitt (Atkins 87 mins), King, Edge. Unused sub: Hayward.