Dimon was born in New York City, one of three sons born to Greek immigrants Theodore and Themis (née Kalos) Dimon. and attended The Browning School in his youth. His paternal grandfather was a Greek immigrant who changed the family name from Papademetriou to Dimon to make it sound more French, and worked as a banker in Smyrna (today's Izmir) and Athens. He has an older brother, Peter, and a fraternal twin brother, Ted. Dimon's father and grandfather were both stockbrokers at Shearson.