Sanborn signed with the Chicago Bears as an undrafted free agent on May 6, 2022.[12] Sanborn eventually made the Bears' 53-man roster for the 2022 season.[13] He became the Bears' starting middle linebacker following the trade of Roquan Smith.[14] On November 13, during his first career start, Sanborn recorded two sacks and 12 total tackles during a 31–30 loss to the Detroit Lions.[15] On December 18, Sanborn suffered a season-ending ankle injury in the Bears' 25–20 loss to the Philadelphia Eagles,[14] and was placed on injured reserve on December 20.[16] He made six starts for the Bears in 2022, where he recorded 64 total tackles and two sacks.[14] Pro Football Focus (PFF) named Sanborn to their 2022 All-Rookie Team following the regular season.[17]
The Bears signed Tremaine Edmunds and T. J. Edwards to reinforce their linebacker corps in the ensuing offseason. Sanborn transitioned to a strong-side linebacker.[18] He started all 17 games of the 2023 Chicago Bears season, where he registered 65 tackles, one interception, one sack, and seven tackles for loss.[19] He was named to ESPN's 2023 ESPN "All-Youngster Team", a collection of players 24 years old and younger.[20] In 2024, Sanborn played in all 17 games while starting in only 3. Sanborn had 35 tackles, 1.5 sacks, and 2 tackles for loss. The Bears announced after the season they would not be tendering Sanborn making him a free agent.[