In the sixth-season finale, a gunman commits mass murder at the hospital. Mark helps Lexie save Alex, her new boyfriend. In the aftermath of the shooting, Lexie has a nervous breakdown and Mark has her committed to the psychiatric unit. Their friendship is strained, however Lexie softens towards him when she learns that he is still in love with her. As Lexie goes to talk to Mark about him still being in love with her, she sees Mark entering his apartment with Derek's sister Amelia kissing her and ultimately sleeping with her. When Callie's girlfriend, pediatric surgeon Arizona Robbins (Jessica Capshaw), breaks up with her, she and Mark have a drunken one-night stand. Shortly thereafter, he reconciles with Lexie, but is delighted when Callie discovers she is pregnant with his child. Lexie, however, is dismayed and angered by the news and leaves Mark once again. Mark takes quickly to fatherhood when his daughter, Sofia, is born.