The Bridesmaids star will play one half of a couple whose son fights with Lily (Aubrey Anderson-Emmons) at kindergarten.
"I can't wait," McLendon-Covey told Entertainment Weekly. "It'll be pretty fun. I usually play someone who hates her kids - this time I love my child!"
Michaela Watkins (Saturday Night Live) will also appear in the episode - the third of Modern Family's fourth season - as the partner of McLendon-Covey's character.
In addition to her Bridesmaids role, McLendon-Covey plays a recurring role on CBS sitcom Rules of Engagement and is currently filming reshoots for ABC's remake of UK sitcom Only Fools and Horses.
The stars of Modern Family recently sued producers 20th Century Fox Television following unsuccessful contract negotiations, but later struck a new salary deal.
> Modern Family exec on Emmys: 'We worry people won't root for us'
> Modern Family kids seek pay rise