Obstfeld published his first novel, The Goulden Fleece, when he was 24. His second novel, Dead Heat, was nominated for an Edgar by the Mystery Writers of America. Although he has published extensively under his own name, he has also written under the pseudonyms Pike Bishop (the Western series Diamondback), Jason Frost (the futuristic series Warlord), Carl Stevens (a mystery series), Don Pendleton (the Executioner series), and Laramie Dunaway (Hungry Women, Borrowed Lives, and Lessons in Survival). He wrote the award-winning young-adult novel Joker and the Thief.

As a screenwriter, Obstfeld adapted his novel Dead Heat for Michael Keaton and his novel Warlord for Arnold Schwarzenegger and Robert Hamner. His original scripts include various genres, from romantic comedy (Mr. Moonlight) to caper-comedy (Foolproof) to teen action (The Joker and the Thief) to cop dramas (Tangled Up in Blue and Gambol’s Luck). He has rewritten original screenplays for Paramount (Sword Fight) and Don “the Dragon” Wilson (Whatever It Takes). Most recently he has rewritten the scripts Whackers and Robodog for Thornbush Entertainment. His adaptation of his novel, Joker and the Thief, is being developed by Chartoff Productions.

He is a contributing editor for Writer's Digest. He has also published two instructional books on writing, The Novelist’s Essential Guide to Crafting Scenes and Fiction First Aid. In addition, he has several books due in the coming years, including What God Wants: What the World’s Major Religions Teach about Today’s Most Controversial Issue, SpiritWise: The Moral Teachings of Native Americans, and Black Op. Four books have been written for Lucent Books: Napoleon Bonaparte, Moby-Dick: Critical Essays, The Renaissance and Nations in Transition: India.