Joely Richardson was originally intended to play the lead role in the series but decided to leave the project to spend time with her family.
Entertainment Weekly says that Tierney has now joined the cast after being in talks for the role since May.
She will play Kathryn Peale, who works in the Manhattan District Attorney's office. The series also stars Numb3rs actor Rob Morrow as a criminal attorney.
ABC picked up The Whole Truth last month.