Noted for his unusual combination of size and speed, he was recruited by the Missouri Tigers, where he played from 1975 to 1978, ranking among the team's reception leaders during his last two seasons.

After a promising rookie season (1979) was ended by injury, Winslow became a key figure in head coach Don Coryell's offense from 1980 to 1983, twice leading the league in receptions.

[9] The University of Missouri recruited Winslow to play football for the Tigers; head coach Al Onofrio described him as "the best looking future prospect on the East side."

Head coach Don Coryell explained that the Chargers had an area of need at tight end, as their current starters Bob Klein and Pat Curran were both in their 30's and likely to retire soon.

Coryell described him as "a big strong man who has great potential and ability," and suggested Winslow could play both tight end and wide receiver.

[28] When the season began, Winslow did serve in both roles, being both the second tight end behind Klein and a third wide receiver behind Charlie Joiner and John Jefferson depending on the formation.

[37] Due to a series of minor injuries, Winslow was unable to win the place in preseason, with Greg McCrary beginning the regular season as the starter.

[40] As the season progressed, Winslow established himself as a key component of the Air Coryell offense and eventually led the league with 89 catches,[41] setting a new NFL record for tight ends by breaking the previous mark of 75 held by Mike Ditka.

[50] McCrary attempted to play through the injury the following week, but Winslow still had only 3 catches for 42 yards,[51] and San Diego lost the AFC Championship game 34–27 to Oakland.

He drew more double coverage after Jefferson was traded to the Green Bay Packers; Winslow also theorized that teams were more aware of the threat he posed following his performances the previous year.

[53] Aided by the midseason arrival of wide receiver Wes Chandler in place of Jefferson,[54] Winslow saw more single coverage as the season went on and produced better numbers.

[g] He achieved these statistics despite repeatedly having to leave the game with dehydration and severe cramps; he was also treated for a pinched nerve in his shoulder and received three stitches in his lower lip.

"[70] Dolphins' tackle Bob Kuechenberg said the game made Winslow's career in a 2017 interview, adding "I deeply resented the fact that they literally carried him off the field five times and he'd come back and do some other miracle.

[74] Winslow expressed impatience with the process, claiming that players' union director Ed Garvey and management negotiator Jack Donlan were both on "ego trips."

While Garvey was seeking a wage scale for the players, Winslow believed that abolishing the NFL draft and encouraging the development of the rival United States Football League (USFL) would do more to increase their salaries.

[77][78] San Diego qualified for the playoffs for the fourth consecutive season, taking their place in an expanded 16-team Super Bowl tournament as the AFC's fifth seed and travelling to the Pittsburgh Steelers for their first game.

Winslow was unable to repeat his performance of the previous year, playing with a twisted ankle for most of the game and finishing with only one catch for 18 yards as Miami won 34–13.

[87] He finished the year with 88 catches for 1,172 yards and 8 touchdowns,[88] and was voted to another Pro Bowl, though this time as the AFC's reserve tight end behind Todd Christensen of Los Angeles.

[91] He requested an amount which the Chargers refused, with Winslow's agent Jim Steiner stating shortly before the regular season started that the two sides were far apart with their offers.

[92][h] Steiner declared Winslow's intention to play in week 1 at Minnesota, "out of respect for the team, Don Coryell and the coaching staff," then retire if no agreement was forthcoming.

He criticized ownership for letting players such as Jefferson and Fred Dean leave in the past due to similar contract disputes, saying "I feel cheated... they've turned their back on me.

[98] Winslow stated at the conference that he would return to San Diego and play the remaining two years there; he was not permitted to sign for the Gamblers until his Chargers contract expired, but expressed an interest in joining them for the 1986 season.

"[111] Once medical staff reset his knee, Winslow smiled and exchanged jokes with the Raiders as he left the field, but learned in the treatment room that his injury would require season-ending surgery after the game.

[115] Reflecting on his season the following week, Winslow was relieved that he had insisted on a guaranteed contract during negotiations, which ensured he would be paid in full for five years even if he was unable to play again.

[133] His performances earned him his fifth Pro Bowl nomination (his first since the knee injury) and a place in the AP All-Pro second team, while his teammates named him their offensive player of the year.

Describing his physical state during his preseason appearance as "total ineptness," Winslow said he had informed Saunders and director of football operations Steve Ortmayer that he was unable to continue his career.

[146] Interviewed the following year, Winslow described his career as "like a fairy tale," saying that he had been fortunate with his choice of high school and college, and with having a head coach in Coryell who could best exploit his abilities.

"[155] Saunders described Winslow as "a wide receiver in an offensive lineman's body,"[152] while Coryell said, "If we're asking Kellen to block a defensive end and not catch passes, I'm not a very good coach.

[93] He earned a Juris Doctor degree from the University of San Diego in 1993 and spent some time as a sports agent, while doubling as a radio commentator for his alma mater the Missouri Tigers.

[170] Winslow has been a vocal proponent of affirmative action, particularly in football—he dedicated a substantial section of his Hall of Fame induction speech to highlighting the lack of African Americans in non-playing roles.