The 20-year-old has made just six appearances for the Easter Road side since joining in the summer from Inverness Caledonian Thistle with an ankle injury reducing his opportunities.
Speaking to Kilmarnock’s website Mackay said: “It’s a good opportunity for me to come to Kilmarnock with the goal of winning the league and hopefully I can help the team do that.
“I’m happy that the manager was keen to bring me in and I can’t wait to get started now. I’m direct, quick and can score goals as well so hopefully I have all the ingredients to show the fans that I can do well on the park.”
Hibs are keen for Mackay to get more first-team experience and feel that by sending him to a club chasing promotion under a manager who has previously worked with and helped to mould younger players he will kick on ahead of his return to the Capital in the summer.
Killie currently sit third in the Scottish Championship, two points off leaders Arbroath but with a game in hand. Mackay joins ex-Easter Road quartet Stephen McGinn, Fraser Murray, Jason Naismith, and Oli Shaw in Ayrshire.
Mackay trained with his new team-mates on Thursday morning and could make his debut for Kilmarnock in Friday night’s Scottish Championship meeting with fifth-placed Partick Thistle.