Dellis was born in Wimbledon, UK, to a Belgian mother and a French father, and grew up in England, France, and the United States. He attended high school at Gulliver Preparatory School in Miami, Florida. After graduation, he went on to attend the University of Miami, where he graduated with a major in physics and a minor in mathematics. He then earned a master's degree in computer science, also from the University of Miami, and wrote his thesis on Automated reasoning. He is a mountain climber who has made three attempts to climb Mount Everest, and has scaled Alaska's Mount McKinley along with other mountains around the world.