The deal, which runs through 2019, was agreed to in principle and announced on Nov. 30.
The contract has buyout clauses for both Sumlin and the school. Sumlin, whose name has been associated with several recent high-profile coaching jobs, must pay the school $5 million if he leaves before the end of the 2016 season. The school's buyout clause runs the length of the contract.
Sumlin is 19-6 in two seasons with the Aggies (8-4) heading into the Chick-Fil-A Bowl on Dec. 31. In six seasons as a head coach at Houston and Texas A&M, Sumlin is 54-23.