The role will mark the small-screen follow-up after his Emmy-nominated turn in 2014 crime-drama anthology "Fargo", said The Hollywood Reporter.
"The Trial" centres on Billy McBride (Thornton), a once respectable lawyer who was ousted from the high-profile firm he co-founded and later gets invigorated professionally when a young lawyer brings him a wrongful death case that pits him against the head of his former firm.
Written by Kelley and his Boston Legal/Practice cohort Jonathan Shapiro, the project will be Amazon's first straight-to-series order.