Not selected in the 2017 NFL Draft, Hollister was soon signed by the New England Patriots on April 30 to a three-year, $1.67 million contract that included $90,000 guaranteed and a signing bonus of $10,000. The Patriots also signed his twin brotherCody as an undrafted free agent; they became the only active pair of twins on the same team in 2017 and the third set of twins to currently play in the NFL. In Week 1 of the 2017 preseason, Hollister caught seven passes for 116 yards against the Jacksonville Jaguars. Hollister put together a solid preseason performance while competing with James O'Shaughnessy for the Patriots possible third tight end spot behind veterans Rob Gronkowski and Dwayne Allen. Hollister ultimately won the spot as an undrafted rookie, and made his NFL debut in Week 2 against the New Orleans Saints, recording his first career catch, which went for seven yards. Hollister made it to Super Bowl LII, but the Patriots lost 41–33 to the Philadelphia Eagles. Hollister entered the 2018 season as the third tight end on the depth chart, behind Gronkowski and Allen. He was limited to just eight games while dealing with hamstring and chest injuries, and was placed on injured reserve on January 8, 2019, prior to the Patriots' postseason. They reached Super Bowl LIII in February in Atlanta and defeated the Los Angeles Rams 13–3.
Seattle Seahawks
On April 29, 2019, the Patriots traded Hollister to the Seattle Seahawks for a seventh-round pick in the 2020 draft. He was waived on August 31, 2019, then signed to the practice squad the next day. Six weeks later, he was promoted to the active roster on October 12. On November 3, he caught a 10-yard walkoff touchdown pass against the Tampa Bay Buccaneers in overtime to give the Seahawks the win, 40–34. Earlier in the same game, Hollister scored the first touchdown of his NFL career, a one-yard reception late in the first half, after drawing a pass interference penalty in the end zone. In Week 10 against the San Francisco 49ers, Hollister caught 8 passes for 62 yards and a touchdown in the 27–24 overtime win. Overall, in the 2019 season, Hollister finished with 41 receptions for 349 receiving yards and three receiving touchdowns. On March 16, 2020, the Seahawks placed a second-round restricted free agent tender on Hollister. He signed the one-year contract on April 21, 2020.