Boyd struck the opener after 16 minutes when he took advantage of Danny Wilson's slip before dummying Jordan McGhee in the box before firing past Jambos keeper Jamie MacDonald.
The second goal arrived 19 minutes after the restart as the former Scotland striker gathered Jeroen Tesselaar's long free-kick after Brad McKay was caught ball watching underneath.
Boyd's first touch took him towards the by-line but his clipped finish over the keeper summed up just how confident the former Rangers man is after six goals in 10 outings this term.
Boyd has not represented his country since featuring against Lichtenstein in September 2010 but Johnston says the 30-year-old is displaying the type of performances that merits a recall.
"I thought Boydy was exceptional," said the Rugby Park manager, whose side have now backed up their first win of the season against Ross County last week with another three points.
"When he plays like that he is unplayable. It wasn't just his goals, it was his work-rate as well. In the last five minutes, you saw him running back to win the ball back for the team.
"Scoring goals is the hardest thing to do and he does it consistently. I don't think there are many people who can do it as well as he can."
It was a bruising encounter that left both teams tending to casualties as Ryan Stevenson limped off with a knee injury for Hearts before Kilmarnock defender Darren Barr left the field on a stretcher with a worrying head injury.
Stevenson has already missed two months of action this season after tearing knee ligaments and Locke confirmed his fresh knock was to the same joint as he said it "looks like a bad one" before admitting he could face a similar spell on the sidelines.
As for Barr, Johnston said: "I'm not sure how he is, he is away to the hospital, so we will have to wait and see.
"It looked a bad one as he came off on a stretcher wearing a neck brace. He was conscious when he came off, so hopefully it was just a precaution."
- Sports & Recreation
- Disease & Medical Conditions
- Danny Wilson
- Jeroen Tesselaar