Cullen was born and raised in Texas, growing up along the Mexican border in El Paso. In 1976, he moved to Santa Fe, where he attended and graduated from St. John’s College. After law school at the University of Texas, he moved back to Santa Fe in 1984. A friend told Cullen about Quivira Coalition shortly after its formation. A short time later, he helped Quivira to acquire Red Canyon Reserve, and with Steve Carson, he has co-managed it ever since.
Cullen met his wife Jone on the back of an elephant in 1968, but postponed marriage until after she retired as a professional ballet dancer. They are still together. Cullen is an avid cyclist, cross country skier, and river runner, and has practiced taijijuan for more than 35 years.