Johnson averaged 7.8 points and 3.8 rebounds in the Las Vegas league. The contract was announced on Tuesday.
The 6-foot-8 Johnson played last season with Zhejiang Chouzhou in China. He averaged 26 points and 9.9 rebounds in 24 games.
Johnson played 125 games over two seasons with Atlanta ending in 2013. He has career averages of 6.5 points and 3.9 rebounds.
The 30-year-old Johnson started his pro career in the NBA Development League with Rio Grande Valley and Anaheim.