The 24-year-old British actress will essay the role of a drug addict in the Yann Demange directorial, according to The Hollywood Reporter.
McConaughey, 47, is leading the ensemble cast that includes Bruce Dern, Jennifer Jason Leigh and Rory Cochrane.
"White Boy Rick" is a true-life story of Richard Wershe Jr who became an undercover informant at the age of 14 before becoming a major dealer until he was arrested at age 17.
However, the actor who will play Wershe has not yet been cast.