The "Hidden Figures" star will voice Peg, a dog Lady befriends when she is taken to the pound.
Lady will be portrayed by Monae's close friend Tessa Thompson, while Justin Theroux will voice the Tramp in the revamp of the animated 1955 classic.
In the classic feature, Tramp is a streetwise hound who falls for pampered pooch Lady. The unlikely couple tackle a series of adventures and famously share a plate of spaghetti and meatballs in the beloved Disney film's most iconic scene.
The new movie is set as one of the big draws on Disney's highly-anticipated Streaming Service, which debuts in 2019. It will also star Thomas Mann, Kiersey Clemons, Benedict Wong, and Ashley Jensen.
The "Lady and the Tramp" casting news emerges days after Janelle also landed roles in the Harriet Tubman biopic and animated movie "UglyDolls".