Nicknamed "Birdman",[1] Andersen was born in Long Beach, California, grew up in Iola, Texas, and played one year at Blinn College.

He received a two-year ban from the NBA in 2006 for violating the league's drug policy,[3] but was reinstated on March 4, 2008, and re-signed with the Hornets the next day.

Andersen could not get the grades to attend the University of Houston, but went to Blinn College in Brenham, where Stewart's father, Ernest, once served as athletic director.

He played one season with the Blinn Buccaneers, leading the National Junior College Athletic Association players in blocks.

[5] Convinced that he could play professionally, Andersen dropped out of Blinn in 1999, not knowing he had to officially apply for the NBA draft to get picked up.

[2] Later that year, Andersen joined the Dakota Wizards of the International Basketball Association (IBA) but left before the season started.

[7][8] Andersen became the first D-League player called up by an NBA team, signing with the Denver Nuggets on November 21, 2001, after just two games for Fayetteville.

[10] During the 2002 Rocky Mountain Revue, teammates Junior Harrington and Kenny Satterfield nicknamed Andersen "Birdman" for his arm span and penchant for aerial acrobatics.

[3] As Andersen waited for his reinstatement, effective January 2008,[15] he was helped by a lawyer friend in Denver, Mark Bryant, who became his adviser.

Andersen spent a month in a rehab clinic in Malibu, worked out and coached a boys' basketball team in Denver.

[17] The reinstatement was effective immediately, and the rights to his services belonged to his former team, the New Orleans Hornets, who re-signed him on March 5, 2008, for the rest of the 2007–08 season.

[22] Nuggets general manager Masai Ujiri, a friend of Andersen's, reluctantly made the transaction in order to remove $9 million from the team's payroll cap to avoid the luxury tax.

[23] Before he was waived by the Nuggets, Miami Heat head coach Erik Spoelstra had repeatedly lobbied Pat Riley to acquire Andersen.

[35] Three days later, he made his debut for the Grizzlies in a 109–104 win over the Minnesota Timberwolves, recording four points, three rebounds and one block in 11 minutes.

[38] On February 13, 2017, the Cavaliers traded Andersen and cash considerations to the Charlotte Hornets in exchange for a 2017 protected second-round pick.

In September 2013, it was revealed that Andersen, along with Internet model Paris Dunn, were victims of an elaborate catfishing hoax, orchestrated by Canadian woman Shelly Chartier in Easterville, Manitoba.