The 24-year-old Johnson most recently played for Rio Grande in the NBA Development League, where he averaged 20.1 points and 5.6 rebounds in 17 games. The 6-foot-6 Johnson began the season in the NBA with Philadelphia, where he played nine games before being waived Nov. 15.
He played two games with Utah in early February.
Johnson's most extensive NBA playing time came last season in Boston, where he averaged 6.3 points in 40 games.
The Bucks return home Saturday to face Washington after going 0-4 on a Western Conference road trip.