The 46-year-old actress portrays Agent Isabelle Hartley, a veteran agent who helps Director Phil Coulson (Clark Gregg) rebuild the organization after it is infiltrated by Hydra.
"When we meet her, she might not necessarily be on the right side of things," executive producer Jed Whedon recently told Entertainment Weekly.
Lawless is no stranger to action roles, having starred on Hercules: The Legendary Journeys and its spinoff, Xena: Warrior Princess. She has also appeared on Battlestar Galactica, Spartacus: Blood and Sand and Spartacus: Vengeance.
Agents of S.H.I.E.L.D. premiered on ABC in 2013, and centers on Director Phil Coulson. The character appeared to die in Marvel film The Avengers (2012), but the television series posits he survived and continued to oversee S.H.I.E.L.D. operations.
Other new cast members include Adrianne Palicki as Mockingbird, Kyle MacLachlan as Skye's (Chloe Bennet) father, Reed Diamond as Hydra villain Daniel Whitehall, aka Kraken, Henry Simmons as Agent Alphonso "Mac" Mackenzie, Simon Kassianides as Bakshi and Nick Blood as a mercenary.
Season two of Agents of S.H.I.E.L.D. premieres September 23 on ABC.