The Pittsburgh Steelers selected Johnson 66th overall in the third round of the 2019 NFL Draft.[9] The Oakland Raiders received Antonio Brown in exchange for the selection when the Steelers first got it. Against the New England Patriots in Week 1, Johnson made his NFL debut. Johnson had three receptions for 25 yards during the 33–3 defeat.[10] The Steelers fell 24–20 to the San Francisco 49ers in Week 3, despite Johnson catching three catches for 52 yards and his first career score.[11] Johnson caught five catches for a career-high 84 yards in Week 8 against the Miami Dolphins, including a 45-yard touchdown in the team’s 27-14 victory.

Johnson suffered a concussion in Week 11 during Thursday Night Football against the Cleveland Browns after safety Damarious Randall struck him helmet to helmet.[13] Johnson won AFC Special Teams Player of the Week in Week 14 against the Arizona Cardinals after he caught six passes for 60 yards and a touchdown and returned a punt for an 85-yard touchdown in the team’s 23-17 victory.[14][15] Johnson was selected as a punt return specialist for the NFL All-Pro Second team on January 3, 2020.[16] Johnson amassed 59 receptions for 680 receiving yards and five receiving touchdowns by the end of the 2019 campaign.

Johnson caught 88 passes for 923 yards and seven receiving touchdowns in 15 games before the end of the 2020 campaign. Throughout the season, he went over 100 yards in two different games.[18]

Johnson caught 11 passes for 117 yards in the 48–37 loss to the Browns in the Wild Card Round of the playoffs.