Fells attended UC Davis (Davis, CA) and joined the team as a wide receiver. Following a position change orchestrated by the coaching staff, Fells made the transition to play tight end and as a senior was a Second-team All-GWFC selection with 10 receptions for 165 yards and one touchdown. In 2004, as a junior, he was a First-team All-GWFC, with 35 receptions for 520 yards and one touchdown. As a member of the UC Davis Aggies in 2003, Fells won team’s award as outstanding sophomore. In 2002, Fells was on the roster but did not play, and he redshirted in 2001.

Fells was originally signed by the Atlanta Falcons as an undrafted free agent in 2006.

In October 2008, Fells signed a free agent contract with the St. Louis Rams. In 2008, Fells played 12 games with one start and caught 7 passes for 81 yards. He was placed on injured reserve on December 21, 2008. In 2009, he came back with 21 catches for 273 yards and 3 touchdowns. He played in 14 games and started 4. Then in 2010, he played in every game and started in 7. He had career high in catches, and receiving yards with rookie Sam Bradford.

Fells signed a one-year contract with the Denver Broncos on July 31, 2011.

Fells agreed to sign a three-year deal with the New England Patriots on March 19, 2012. On August 30, 2013 Fells was released.

On January 7, 2014, Fells was signed by the New York Giants.

On October 5, 2015, the Giants announced that Fells had been dealing with a chronic ankle condition, which was later discovered to be a staph infection, a discovery made after he had received a cortisone injection for his ankle injury. Specifically, it was found to be MRSA, which is resistant to many antibiotics that are used to treat staph infections. He has had six surgeries on his foot. Doctors do not want to amputate his foot, but they are afraid that the MRSA infection may have spread, or may spread, to the bone (osteomyelitis), or even the blood (sepsis), both of which could ultimately be fatal even with proper medical care. Fells had had a 104 degree F temperature upon his arrival on October 2, 2015, at the emergency room, where he had been taken by his wife. Due to the staph infection found in his ankle, he was placed on injured reserve. As of October 9, 2015, Fells’ condition appeared to possibly be improving, as he was moved from the intensive care unit (ICU) to a private room. On July 26, 2016, Fells announced his retirement from the NFL.