Ruff was born in New York City in 1965 to a Lutheran family of German ancestry. His father was a hospital chaplain, and his maternal grandfather was a missionary. At the age of 5, he decided he wanted to be a fiction writer. He spent his childhood and adolescence learning how to tell stories. In elementary school, he wrote a number of short stories, many of them starring his classmates in scenarios cribbed from movies or TV. Ruff has said that reading these aloud in English class was his first experience performing in front of an audience and his first solid evidence that he had what it took to entertain people with his storytelling. Many adults around him attempted to persuade him to choose a different career, but Ruff's mother was supportive of his hope to become a writer; for one of Ruff's birthdays, she bought him an IBM Selectric typewriter. From third to eighth grade, Ruff attended a parochial school. He attended Stuyvesant High School in Manhattan; one of his teachers there was the memoirist Frank McCourt.