The Lions waived Johnson on Thursday, officially cutting ties with their 2018 second-round pick out of Auburn.
Johnson joins a suddenly crowded Eagles backfield that includes Miles Sanders, rookie Kenny Gainwell, Jordan Howard and Boston Scott. Sanders is expected to be the lead back, with the rest of the group competing for a spot in the rotation.
Philadelphia selected Gainwell, a dual-threat back out of Memphis, in the fifth round of the 2021 NFL draft.
Johnson became expendable after the Lions drafted D'Andre Swift No. 35 overall in 2020, signed free-agent running back Jamaal Williams in March and then drafted Oregon State's Jermar Jefferson in the seventh round this year.
Johnson was productive in his first season with Detroit, leading the team in rushing (641 yards) in 2018 despite playing just 10 games because of a knee injury. But another injury kept him to just eight games in 2019, and Swift and veteran Adrian Peterson ultimately took over the backfield duties this past season.
Overall, Johnson, who turns 24 in June, ran for 1,225 yards and eight touchdowns over three seasons in Detroit. He also had 61 receptions for 527 yards and three scores.
ESPN's Tim McManus contributed to this report.