When the San Diego Chargers selected Keenan Allen in the third round (76th overall) of the 2013 NFL Draft out of Cal, few predicted how quickly the smooth route-running specialist would seize control of the team's aerial attack. Stepping into a prime role as a rookie, Allen erupted for 71 receptions, 1,046 yards, and eight touchdowns, earning PFWA All-Rookie Team honors and instantly becoming Philip Rivers’ most trusted target.

Despite that electric start, Allen soon ran into a brutal string of bad luck. After a quieter sophomore year, he was on pace for a career season in 2015, logging 67 catches through eight games before a severe kidney injury cut his campaign short. Misfortune struck again in the 2016 opener when he suffered a season-ending torn ACL, raising questions about his long-term durability.

Allen answered those doubts with a triumphant performance in 2017—the franchise's inaugural year in Los Angeles. He played all 16 games, catching 102 passes for 1,393 yards and six touchdowns to sweep AP and PFWA Comeback Player of the Year honors and earn his first Pro Bowl selection. That launched a dominant five-year Pro Bowl streak through 2021. In 2023, he set a single-season Chargers record with 108 receptions for 1,243 yards, earning his sixth Pro Bowl nod.

Contract negotiations led to a trade to the Chicago Bears for the 2024 season, where Allen posted 70 catches for 744 yards and seven touchdowns. But the separation from Southern California didn't last long. In August 2025, Allen returned home to the Chargers on a one-year deal. His reunion season proved historical: in Week 5 against Washington he became the fastest player in NFL history to reach 1,000 career receptions (159 games), and in Week 10 against Pittsburgh he officially passed Antonio Gates as the Chargers' all-time career receptions leader. He finished his 2025 homecoming season with 81 catches for 777 yards and four touchdowns.  He joined the Indianapolis Colts for the 2026 season.

With the Bolts, Allen caught 985 passes for 11,307 yards and 63 touchdowns.