Manziel Wednesday confirmed he would bypass his final two years of college eligibility and make himself available for this spring's draft.
Manziel is projected as a first-round choice in the May 8-10 draft.
Manziel posted a letter to A&M's football fans stating his plans to leave the school
After promising to "always be an Aggie," Manziel wrote, "I will always try to make you as proud of me in the NFL as I did at Texas A&M."
He thanked the fans for "making my college years very special."
Manziel, 21, made college football history in 2012 when he won the Heisman Trophy after his freshman season. Manziel, despite having an arguably better season statistically in 2013, finished fifth in the Heisman voting this past season.
Florida State quarterback Jameis Winston, another freshman, won the honor for 2013.
Manziel passed for 7,820 yards with 63 touchdowns in two seasons with the Aggies and rushed for another 2,169 yards and 30 touchdowns.
A&M was 20-6 with Manziel as the starting quarterback, finishing this past season ranked No. 18 after a 9-4 season that ended with a thrilling 52-48 win over Duke in the Chick-fil-A Bowl.