Fortson began his pro career after being drafted 10th overall by the Milwaukee Bucks after a three-year collegiate career at the University of Cincinnati. He was immediately traded the same day to the Denver Nuggets. In his first year, he averaged double digit points and the next year averaged a double-double in both points and rebounds. Before the 1999-2000 season, he was once again traded but to the Boston Celtics in a six-player deal. He missed the first 25 games of the season with a stress fracture in his right foot.
Fortson did not play a lot with the Celtics and only averaged 7.6 points and 6.7 rebounds per game. Following that season, he was traded to the Golden State Warriors in a four-team trade. He averaged 16.7 points and 16.3 rebounds per game for the Warriors in the first six games of the 2000-2001 season, but those were the only games he would play due to another stress fracture injury in his right foot. He returned to form the following season, but his performance was not nearly as strong as it was the previous year. In the 2002-2003 season, Danny mostly spent his time on the bench as most of his minutes went to Antawn Jamison and Troy Murphy. For the fourth time in his career, Fortson was again traded. This time, in a nine-player deal to the Dallas Mavericks.
Fortson only averaged 11 minutes per game for the Mavericks, having to play behind forwards Dirk Nowitzki and Antoine Walker. Because of a need for a center who could make a jumper, the Mavericks swapped Fortson for Calvin Booth with the Sonics in the offseason of 2004. Quickly, Danny was back in the rotation and became a fan favorite in Seattle because of his physical play and rebounding prowess.
Fortson is a frequent target of foul calls by referees because of his physical play. As a result, Fortson registered the second highest technical foul total among all active players during the 2004-2005 season.
After returning from a two-game suspension on January 02, 2006, Fortson called NBA senior vice president of basketball operations Stu Jackson a "gangster" for the nearly $200,000 in fines Fortson was docked for his latest transgression with an official.