Trelawny's birthplace is unknown, although he claimed that he was born in Cornwall. Some of his biographers have contended that he was actually born in London, as he was baptised in Marylebone as John Trelawney on 29 November 1792. His father, Charles Trelawny-Brereton (a descendant of Sir Jonathan Trelawny, 2nd Baronet, who married Mary, daughter of Sir Edward Seymour, 2nd Baronet, a descendant of Edward Seymour, 1st Duke of Somerset) was a British military officer who retired after reaching the rank of lieutenant colonel. His mother, Maria Hawkins, was of Cornish descent. She had inherited a small sum of money from her father, but the family soon spent most of the inheritance. When Edward was six years of age, his father inherited a large sum of money from one of his wealthy cousins. The family soon used the inheritance to move to London, where they lived in a large house. As a condition of receiving the inheritance, his father was required to adopt the last name "Brereton". His cousin, Owen Brereton, wished to have his last name continued along with his wealth. They lived in Cheshire while Edward was a young man.