Johnson came to Iowa after starting his collegiate career in his hometown at Tyler Community College in Texas. Known as a good outside shooter, Johnson was 2nd in the Big Ten in three-point field goal percentage his Junior season. His Senior year, Johnson started every game while averaging 11.8 points per game and 4.9 rebounds per game. Twice during his final year with the Hawkeyes, Johnson hit 8 three-point baskets in a game. When his collegiate days came to an end, Johnson began his next career in the NBA D-League with the Sioux Falls Skyforce. After playing only sparingly in the 2008-2009 season Johnson went overseas the next year and signed with CS Municipal Bucuresti in Romania. He averaged 15.2 points, 5.3 rebounds, 1.4 assists and 1.0 steals per game. He left Romania and began playing with Jiangsu in China. There he averaged 17.7 points, 8.6 rebounds, 2.6 assists and 1.7 steals per game while helping his team to a League Championship. He left China and for the 2010-2011 season, Justin played in Japan with the Tokyo Apache. He averaged 6.3 PPG, 6.0 RPG and 3.3 APG before the earthquake & tsunami ended the season prematurely. Johnson then rejoined Jiangsu in May after being drafted with the 10th overall pick of the NBL League Draft. He averaged 13.2 points, 8.3 rebounds, 2.2 assists & 1.2 blocks per game. Justin came back to North American and went North of the border, joining the Halifax Rainmen. He played in 44 games for the Rainmen while averaging 10.0 points and 4.8 rebounds per game. In 2012, Johnson was drafted in the NBA D-League draft by the Austin Toros. He never played for them after heading over to Japan and signing with the Fukuoka Rizing.