With the two fierce rivals locked at 18-18 the visitors broke down the left.
Knight picked up at the back of a ruck, stood up his defender to leave him on his backside and ran in to score.
The Havant captain added the conversion to give his side a hard-fought win.
Earlier Knight had also kicked a tremendous long-range penalty from just inside the Tottonians half.
Senior coach Will Knight felt the contest was every bit as competitive as expected.
He said: ‘Totts came at us like a wounded animal having just been relegated. It was a scrappy hotly-contested affair.
‘They were slow getting to line-outs and set-pieces which was very frustrating.
‘We wanted to play the game at a much quicker tempo.
‘Once again it showed just how competitive it is in this league and there are few, if any, easy games.’
Tottonians were quicker out of the starting blocks and raced into a 15-0 lead.
Havant hit back with their opening try courtesy of forward Joe Davis with Knight soon adding a penalty.
Wayne Dugan made sure the two sides went in level at the break with a try wide out on the left hand side.
Despite applying all the pressure at the start of the second period the visitors found themselves trailing again.
A break out saw the home side rewarded with a penalty.
Joel Knight responded in kind with his long-range effort to set up the tense finale. And Havant went on to edge the win.
Will Knight took little joy in seeing the home side relegated.
He added: ‘We have had some great encounters over the past few seasons. It is a shame to see them go but hopefully they will bounce back up soon enough.
‘Both clubs have a tremendous respect for each other and the rivalry, though intense, is also a friendly one.’
Havant will bring the curtain down on their season against Medway at Hooks Lane on Saturday.
It promises to be another great occasion with the Kent side needing to win if they are to stand a chance of promotion.
Havant will be equally keen to finish on a winning note in front of their own fans.
‘We will very much be up for this one,’ said Will Knight.
‘Our aim is to head into the summer on a positive note.’