In 11 seasons with the Chargers, Allen was a six-time Pro Bowler and is one of two players in franchise history to accumulate more than 10,000 career receiving yards.

Allen was regarded as a five-star recruit by Rivals.com and Scout.com,[3] and was widely considered the top defensive back in the nation.

[11] He left Cal after his junior season having caught a school-record 205 passes, for 2,570 yards (third in school history) and 25 touchdowns (seventh).

[12] Due to a posterior cruciate ligament (PCL) sprain he sustained during the 2012 season, Allen did not work out at the NFL Scouting Combine, but did run routes at Cal's Pro Day.

On April 9, 2013, Allen attended his own pro day, where he showed his route-running skills and catching abilities to numerous scouts and reporters.

Allen was selected in the third round, 76th overall, by the San Diego Chargers in the 2013 NFL draft, though the team already had solid depth at the wide receiver position.

[19] In 2011, Telesco attended Cal's game against USC at San Francisco's AT&T Park where Allen enjoyed one of the most prolific days of his career that evening, catching a career-best 13 passes for 160 yards.

Teammate and veteran tight end Antonio Gates envisioned that Allen in 2013 would play behind Eddie Royal, Vincent Brown, Malcom Floyd, and Danario Alexander.

[16][22] Although Alexander suffered a season-ending injury in the preseason, Allen did not participate in any offensive plays in San Diego's season opening loss to the Houston Texans.

Unaccustomed to being a reserve player, he considered quitting football and returning to school to complete his degree.

[16] He began receiving playing time the following week against the Philadelphia Eagles when Floyd suffered a season-ending injury.

[23] The next week, in a win against the Indianapolis Colts on Monday Night Football, Allen recorded nine catches for 107 yards and a touchdown.

[29] He also set a Chargers rookie record with five 100-yard games in a season, and he tied Royal for the team lead in touchdowns.

[30] Allen was runner-up to Lacy for the Offensive Rookie of the Year by the Associated Press (AP),[31] considered by the NFL to be its official award.

[32] Many experts came to view Allen's being drafted in the third round as a steal by San Diego;[33][34][35] seven other wide receivers were taken before him.

[37] They advanced to the Divisional Round, when Allen caught two touchdowns during a comeback attempt in the fourth quarter of a 24–17 loss to the Denver Broncos.

[46] However, the following week Allen caught both of the Chargers touchdowns and had 12 receptions for 133 yards in a losing attempt against the Minnesota Vikings.

[52] On September 11, 2017, in the season opener against the Broncos on Monday Night Football, Allen returned from his injury and had five receptions for 35 yards and a touchdown in the 24–21 loss.

[55] On Thanksgiving Day, during Week 12 against the Dallas Cowboys, Allen finished with 172 receiving yards and a touchdown as the Chargers won 28–6.

[56] In Week 13, Allen became the first player in NFL history to record three consecutive games of 10+ catches, 100+ receiving yards, and at least one touchdown.

His 1,393 receiving yards finished third in the league behind Antonio Brown and Julio Jones and were second most in a season in Chargers history behind Lance Alworth.

[59] He was named the NFL Comeback Player of the Year after an impressive 2017 season coming back from a torn ACL in 2016.

[95] On March 14, 2024, Allen was traded to the Chicago Bears in exchange for a fourth-round pick in the 2024 NFL draft.

[103] They share a mother, Dorie Maynard-Lang, and Maynard's biological father, Scott Lang, is Allen's stepfather.

[9][103][104] Allen's cousin, Maurice Harris, is a former NFL wide receiver, with the two being teammates in high school and college.

[105] Allen is childhood friend of former cornerback David Amerson, who was selected in the 2013 NFL draft by the Washington Redskins.