Striker McKenzie made his senior debut for Killie in April 2011, but he was overlooked last term and spent the second half of the season on loan with Brechin.
But he returned to Rugby Park and netted his first goal for the club in the 3-1 win over Dundee United last August.
The Scotland U21 international has made eight first-team starts and another 15 appearances off the bench, and scored an impressive individual goal in Saturday's 3-2 victory at Dundee.
Defender Ashcroft, who also came through the youth system at Killie along with McKenzie, has caught the eye with several excellent performances for the club's U20 side this season.
He earned his first-team debut at Dens Park at the weekend, replacing Mohamadou Sissoko for the final 10 minutes of the match.
A club statement read: "Both youngsters are products of the youth system and look set to have an exciting future ahead of them at Kilmarnock Football Club.
"Impressive performances at both senior and U20 level for Killie saw Rory rewarded with a Scotland U21 cap in a 1-1 draw with Greece in February.
"Lee Ashcroft has been a key figure for the U20 side this season in the centre of defence."