Hennessy played in three games with one start at left guard as a true freshman before redshirting the rest of the season. He became the Owls' starting center going into his redshirt freshman year, starting all but one game that he missed due to injury.[5] Hennessy was awarded a single-digit jersey (number three) as one of the nine toughest players on the team going into his redshirt sophomore year, which he wore in practice as offensive line are not eligible to wear single digit numbers in-game. He started eleven games, missing two due to injury.[6][7] Hennessy started 12 games at center for the Owls as a redshirt junior and was named first-team All-American Athletic Conference, second-team All-America by USA Today and to the third-team by the Associated Press and was named a finalist for the Rimington Trophy.[8][9] Following the end of the season, Hennessy announced that he would forgo his final season of NCAA eligibility to enter the 2020 NFL draft.[10] Having graduated in December of his redshirt junior year, Hennessy was invited to and played in the 2020 Senior Bowl.[11]
Hennessy was selected by the Atlanta Falcons in the third round of the 2020 NFL Draft with the 78th overall selection.[14]
After playing behind Alex Mack as a rookie, Hennessy was named the Falcons starting center in 2021 and started every game. He started Week 9 at left guard before suffering a knee injury. He was placed on injured reserve on November 8, 2022.[15] He was activated on December 31 and he started the final two games of the season at left guard.[16]
In 2023, Hennessy suffered a knee injury in training camp and was placed on injured reserve on August 16, 2023.[17]
Philadelphia Eagles
On March 13, 2024, Hennessy signed a one-year contract with the Philadelphia Eagles.[18] He was released on August 27.[19]
Atlanta Falcons (second stint)
On September 24, 2024 with starting center Drew Dalman going to injured reserve, the Falcons signed Hennessy to their practice squad.[20] He was promoted to the active roster on October 15.[21]