Holden will portray Andrea, one of two sisters who survive the story's zombie plague, while Yeun has signed on to play scavenger Glenn.
The duo join Andrew Lincoln, Sarah Wayne Callies, Jon Bernthal and Jeffrey DeMunn in the Frank Darabont directed project.
In Robert Kirkman's original comics, Andrea is a skilled marksman and Glenn's ability to forage for food in the zombie-infested cities makes him indispensable to the team.
The Walking Dead has been commissioned for six episodes. A pilot shoots this month.