Arnold went undrafted in the 2017 NFL Draft, but received invitations to attend rookie minicamp and tryout for the Kansas City Chiefs and New Orleans Saints. On June 5, 2017, the Saints signed Arnold to a three-year, $1.66 million contract. On June 16, 2017, the Saints placed Arnold on injured reserve due to an unspecified injury. Arnold was initially waived/injured, but was placed on injured reserve after clearing waivers. On July 26, 2018, it was reported that the New Orleans Saints' coaching staff had elected to move Arnold from wide receiver to tight end. Arnold was moved in order for him to have a better chance of making the roster. The move was partly due to the release of tight end Coby Fleener, leaving the a lack of depth at the position. Assistant head coach/tight ends coach Dan Campbell oversaw Arnold's transition to tight end and Arnold credits him with his success at tight end. Throughout training camp, Arnold competed to be the third tight end on the depth chart against Michael Hoomanawanui, John Phillips, Garrett Griffin, and Deon Yelder. Arnold impressed the coaches with his ability during training camp and made the active 53-man roster. Head coach Sean Payton named Arnold the third tight end on the Saints' depth chart to begin the regular season, behind veterans Benjamin Watson and Josh Hill. Arnold was inactive for the first four regular season games as a healthy scratch. On October 8, 2018, he made his NFL debut during the Saints’ 43–19 win against the Washington Redskins in Week 5. In Week 7, he made his first career reception, a ten-yard reception from Saints’ quarterback Drew Brees during the second quarter of a 24–23 win at the Baltimore Ravens. He finished the game with two receptions for 35 yards. On November 11, 2018, Arnold earned his first career start and made two receptions for 25 yards as the Saints defeated the Cincinnati Bengals 51–14 in Week 10. On November 22, 2018, Arnold scored his first touchdown against the Atlanta Falcons on a 25-yard reception. On August 31, 2019, Arnold was waived by the Saints and was signed to the practice squad the next day. He was promoted to the active roster on October 19, 2019. On December 4, 2019, Arnold was waived by the Saints.
Arizona Cardinals
On December 5, 2019, Arnold was claimed off waivers by the Arizona Cardinals.
