Shell was selected by the San Francisco 49ers in the first round of the 1984 NFL draft.[2] In Week 13 of the 1984 season, against the New Orleans Saints, Shell became the first NFL rookie to record 2.0+ sacks and a touchdown in a single game, when he returned an interception for 53 yards.[3] As of 2023, only two other defenders, Andy Katzenmoyer and Devon Witherspoon, had achieved this.