In transcribed marriage records from Newfoundland churches, Will (or Wills) Terry Efford married Mary Hillier (Hellier) on December 21 1790. Both were from Petty Harbour, Newfoundland. Mary Hillier was the daughter of Hannah Brace and Richard Hillier, and Richard Brace was a witness at the wedding, which took place in the Cathedral of St John the Baptist in St. John's.
An article on the Efforts of Petty Harbour speculates that Will Terry could have been the son of George Efford and Ann Gempton of Petty Harbour, Newfoundland, born in 1735. This William Efford had a twin sister, Ann.