Cary Elwes was born in Westminster, London, England, the third son of interior designer/shipping heiress Tessa Georgina Kennedy and the late portrait painter Bede Evelyn Dominick Elwes. He is the brother of producer/agent Cassian Elwes and artist Damian Elwes. He was raised in London and attended Harrow. After graduating from Harrow, he moved to the US and studied drama at Sarah Lawrence College. He left school after two years to begin his film career. Cary is well respected by colleagues and fans alike and considered by many to be one of the finest actors working today. He is interested in history and says, "It's deliberate that a lot of my films have been period pieces". He is politically active for causes he believes in, such as protecting the environment and helping Native American people.

Elwes is married to Lisa Marie Kurbikoff, a stills photographer. He comes from a long-established recusant English family on his father's side. Several prominent Catholic clerics are among his relatives, including Fr. Luke Cary-Elwes, Dom Columba Cary-Elwes, and Dom Cuthbert Cary-Elwes. His grandfather was society painter and war artist Simon Elwes. Cary (the surname "Cary-Elwes" was shortened to "Elwes" in some branches of the family) was an altar boy at London's Brompton Oratory, although he did not attend a Catholic high school. From his maternal grandmother, Daska Marija Ivanovic-Banac, who was born in Osijek in the Austra-Hungarian Empire (now in Croatia), he has Croatian Jewish and Serbian ancestry. Cary's other lineage is English, Irish, and Scottish.