Schwahn has co-written Coach Carter (2005), The Perfect Score (2004), Whatever It Takes (2000) and 35 Miles from Normal (1997). In addition, he was creator of the TV series One Tree Hill, for which he also wrote and directed.
He served as producer for Whatever It Takes and One Tree Hill. He also directed 35 Miles from Normal, which he filmed in his hometown of Pontiac, Illinois. In March 2015 E! premièred Mark's show The Royals. E! first scripted series follows the lives of a fictional British Monarchy. The Royals stars Elizabeth Hurley as Queen Helena Henstridge, Alexandra Park as Princess Eleanor, William Mosley as Prince Liam, and Jake Maskall as King Cyrus.
On November 12, 2017, TV writer Audrey Wauchope wrote on Twitter that she and her female writing partner were sexually harassed by Schwahn while working on One Tree Hill. Female cast members supported the allegations, and Hilarie Burton and Danneel Harris alleged that they had also been sexually harassed by Schwahn.
Schwahn was also accused by cast members of The Royals. Twenty-five female cast and crew members of the show released their own open letter stating that they, too, had been subjected to sexual harassment by Schwahn throughout the run of the show