Before 15 Feb 1687 [his father] Brigadier-General Henry Trelawny (age 29) and [his mother] Rebecca Hals (age 26) were married. She a co-heiress of her brother Matthew Hals (d.1684) of Efford, from whom she inherited the manor of Efford. They had two sons and three daughters. Some sources describe the date of their marriage as 08 Jan 1690 which is inconsistent with the dates of birth of their two eldest children Harry Trelawny 5th Baronet and [his sister] Mary Trelawny.

On 26 Mar 1701 [his father] Brigadier-General Henry Trelawny (age 43) and Mary Trevill were married. There was no issue from the marriage.

On 17 Jul 1716 Harry Trelawny 5th Baronet (age 29) and Letitia Trelawny were married. She the elder sister of John Trelawny 4th Baronet (age 24). They had been courting for many years before her father Bishop Jonathan Trelawny 3rd Baronet (age 66) consented to their marriage. They had a son, who predeceased him, and three daughters, one of whom, Laetitia, also married a cousin. They were first cousins.

On 25 Jun 1754 [his son-in-law] William Trelawny 6th Baronet (age 32) and [his daughter] Laetitia Trelawny (age 26) were married at Maker, Cornwall. They had a son and a daughter. They were first cousins.