Boyd struck in the final minute to seal a 2-1 win at Rugby Park as Kilmarnock put their early season woes behind them with another morale-boosting victory.
Johnston said: "If we get chances to Boydy, he will always score. He got one and was unlucky with a couple of others. That's how much of a threat he was.
"Boydy knows how much of a big player he is for us. Hopefully, we can keep hold of our best players. We are looking to improve the team rather than lose our best players."
Kilmarnock, who failed to collect a win bonus from their opening 10 games, have collected three wins out of four over the festive programme.
"The festive period is very important and to take nine points out of 12 is great," Johnston said. "I'm delighted with the results we have been getting.
"It shows you how far the team has come that we came back and won the game after losing the first goal.
"We are not going to get complacent just because we have won a few games. We know we need to improve a lot to get where we want to be.
"Craig Slater never seems to give the ball away and it is always the sign of a good player when it looks as though you have time on the ball."
St Mirren went ahead through on-loan Andy Campbell but were pegged back by McKenzie before Boyd's decisive intervention.
Manager Danny Lennon rued his side's late lapse - but was cheered by the performance of new recruit Campbell.
"That was a sore one to take and we're very frustrated and disappointed with the result," he said.
"You wonder how you lost it but if you give someone as prolific as Kris Boyd an opportunity then he can punish you.
"I thought wee Adam Campbell gave a great flavour of what he's going to bring, not just for us but to Scottish football as a whole. He's got lovely soft feet, a lovely touch and good pace.
"He's going to grow in confidence. It will take him a bit of time to settle into our game up here but the goal will give him added confidence."