The eight-part drama - picked up by the cable network in May - will star McConaughey and Harrelson as two detectives on the trail of a killer in Louisiana.
Source Code actress Monaghan will play Maggie Hart, the wife of Harrelson's character Marin, TVLine reports.
Martin and his partner Rust Cohle (McConaughey) pursue their killer quarry over 17 years, forcing Maggie to make a difficult decision that has devastating consequences.
36-year-old Monaghan is best known for her film work, including appearances in 2005's Kiss Kiss Bang Bang, 2006's Mission: Impossible III and 2007's Gone Baby Gone. Her last regular television role was an eight-episode stint on Fox's Boston Public between 2002 and 2003.
The first season of True Detective has been written by The Killing's Nic Pizzolatto and will be directed by Jane Eyre's Cary Fukunaga. If picked up for a second season, the series will undergo a revamp and feature a new central mystery and cast.
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