Jaglom was born to a Jewish family in London, England, the son of Marie (née Stadthagen) and Simon M. Jaglom, who worked in the import-export Business. His father was from a wealthy family from Russia and his mother was from Germany. They left for England because of the Nazi regime. Through his mother, he is a descendant of Philosopher Moses Mendelssohn. Jaglom trained with Lee Strasberg at the Actors Studio in New York, where he acted, wrote and directed off-Broadway theater and cabaret before settling in Hollywood in the late 1960s. Under contract to Columbia Pictures, Jaglom featured in such TV series as Gidget and The Flying Nun and acted in a number of films which included Richard Rush's Psych-Out (1968), Boris Sagal's The Thousand Plane Raid (1969), Jack Nicholson's Drive, He Said (1971), Dennis Hopper's The Last Movie (1971), Maurice Dugowson's Lily, aime-moi (1975) and Orson Welles' never-completed The Other Side of the Wind.