Ellison redshirted the 2007 season, his freshman year with the Trojans. As a redshirt freshman in 2008, Ellison served as an often-used backup tight end. Overall for the season, while appearing in 9 games, he had 4 receptions for 58 yards and a tackle. He started the UCLA and Penn State games at fullback and split time between there and tight end in those games. He broke his right foot prior to the Oregon game and missed those next 4 contests while recuperating. In 2009, Ellison appeared in all 13 games, serving as a regularly used backup tight end while also making plays on special teams as a sophomore. For the season, he had 6 catches for 41 yards with 1 touchdown, plus made 4 tackles. As a junior in 2010, Ellison started all season at tight end, hauling in 21 catches for 239 yards with 3 touchdowns, and also made 3 tackles. He made 2010 All-Pac-12 honorable mention. The steady Ellison, equally proficient as a blocker and pass catcher, started at tight end for his second season as a senior in 2011, but also saw time at fullback. Ellison was captain of the Trojans in 2011 and an award was created to honor his leadership and determination, called the Rhett Ellison "Machine" Trojan Way Leadership Award.
Ellison was taken in the fourth round of the 2012 NFL Draft, 128th pick overall, by the Minnesota Vikings. He was not expecting to be drafted, and as such was on a river excursion and not watching the draft when the Vikings called him to let him know he was about to be selected; the surprise was such that he cried in joy. Trojan teammate and 4th pick overall Matt Kalil was also selected by the Vikings in the first round. He is the second player of Maori heritage to play for the Vikings since David Dixon. On November 24, 2013, Ellison scored his first career touchdown catch against the Green Bay Packers. He scored the third tight end rushing touchdown in franchise history in 2016.
New York Giants
On March 10, 2017, Ellison signed a four-year, $18 million contract with the New York Giants. He started the year as the backup tight end to Evan Engram. He also took snaps at fullback. On September 10, 2017, in the Giants' season opening 19–3 loss to the Dallas Cowboys on NBC Sunday Night Football, Ellison had one reception for nine yards in his Giants debut. In a week 5 loss to the Tampa Bay Buccaneers, he caught his first touchdown as a Giant. On December 27, 2019, Ellison was placed on injured reserve with a concussion. He finished the season with 18 catches for 167 yards and one touchdown through 10 games.
Retirement
On March 9, 2020, Ellison announced his retirement from football.
Personal life
Ellison is of Māori descent. He is the son of three-time Super Bowl championRiki Ellison, a former USC and NFL linebacker. He is also of Ngāi Tahu descent and is the grandnephew of the first captain of the All Blacks, Thomas Ellison, who led New Zealand on their first tour to Australia in 1893. He is also related to former New Zealand rugby union players Jacob Ellison and Tamati Ellison. Ellison completed a bachelor's degree in international relations from USC and a master's degree in communication management. Ellison is married to fashion model Raina Hein, runner-up of season 14 of America's Next Top Model, and have two children together.