Winslow announced his eligibility for the 2004 NFL draft and was selected by the Cleveland Browns with the sixth overall pick.

During his freshman season, in 2001, he backed up All-American tight end Jeremy Shockey and played largely on special teams, and was one of four true freshmen to play for the Hurricanes' 2001 BCS National Championship team; the three others were future NFL players Frank Gore, Antrel Rolle, and Sean Taylor.

After Shockey's departure for the 2002 NFL draft, Winslow became the starter at tight end and was named a finalist for the Mackey Award and named a first-team All-American by CNNSI.com, setting University of Miami records for a tight end with 57 receptions for 726 yards and eight touchdowns.

When questioned during the media session following the game, Winslow referred to himself as "a ... [sic] soldier", despite never serving in the military.

[5] Despite a slight drop in production during his junior season, in which Winslow caught 60 passes for 605 yards and one touchdown, he won the John Mackey Award as the nation's best collegiate tight end, and was recognized as a unanimous first-team All-American, after receiving first-team honors from the Associated Press and other national selector organizations.

Two games into his rookie season, however, he suffered a broken right fibula, which cost him $5.3 million in incentive bonuses.

On May 1, 2005, Winslow suffered another leg injury when he was thrown from his Suzuki GSX-R750 motorcycle while riding in the Cleveland suburb of Westlake.

Winslow sustained a torn anterior cruciate ligament in his right knee and was placed on the "Physically Unable-to-Perform (Non Football Injury)" list for the 2005 season.

[citation needed] Winslow attended the Browns' 2006 training camp and pronounced himself ready to play.

[11] Winslow underwent microfracture surgery on his right knee January 31, 2007, at the Cleveland Clinic in an attempt to further repair cartilage damage sustained in the motorcycle accident in 2005.

[12] On February 4, Antonio Gates of the San Diego Chargers announced he would not be attending the Pro Bowl due to injury.

Savage responded by suspending him for a week; owner Randy Lerner later apologized to Winslow and rescinded the suspension.

[15] Winslow went on to a record-breaking season with the Buccaneers in his first year with the team, including single-season franchise records for a tight end in receptions (77) and receiving yards (884).

[28] On October 17, 2006, Kellen's half-brother Justin Winslow died, although no cause of death was reported; he was 23 years of age and was found unconscious by his mother.

[29] Winslow was charged with drug possession in January 2014[30] and received a conditional discharge without a guilty finding in the synthetic marijuana case.

[38] On November 4, 2019, Winslow pleaded guilty to the rape of an unconscious teen and sexual battery on a 54-year-old hitchhiker as part of a plea deal.

As part of the agreement, he will automatically be on lifetime parole immediately upon release, and he waived his right to ever appeal any conviction or verdict in either trial.

[39][40] Winslow's defense attorney said that his client suffered from frontal lobe damage and possible chronic traumatic encephalopathy (CTE).

While CTE could not be used as a defense in the trial,[4] brain trauma was cited by his attorneys in requesting the minimum 12-year sentence.