McDermott was born to Diane and Richard McDermott when they were still teenagers (sources differ on whether he was born in 1961 or 1962). His parents separated when he was a young boy and when he was five or six his mother died in a shooting accident. He and his baby sister were then raised by his maternal grandmother for about 10 years, before he went to live with his father. His father married the dramatist and actress Eve Ensler, who adopted McDermott. They are very close, and it was Ensler who encouraged him into acting, and he majored in drama at Fordham University. Dylan is his stage name.
His film credits include Hamburger Hill (1987), Twister (1989), Steel Magnolias (1989), In the Line of Fire (1993), Miracle on 34th Street (1994), Home for the Holidays (1995), Three to Tango (1999) and Wonderland (2003). He also acts on the stage and television (The Practice).
In 1998 he was voted one of the 50 most beautiful people in the world (People Magazine) and one of the 100 sexiest stars in film history (1995, Empire magazine). He is married with a daughter.