Sylvanus Holbrook, Jr. was born circa 1715 in Mendon, Worcester County, Massachusetts to Sylvanus Holbrook (1685-1740) and Naomi Cook (1693-1762) and died 1782 Massachusetts of unspecified causes. He married Thankful Thayer (1725-1798) 14 April 1748 in Mendon, Worcester County, Massachusetts.
Children
| Name | Birth | Death | Joined with |
| Sylvanus Holbrook (1750-1828) | |||
| Ruth Holbrook (1752-1781) | |||
| Rachel Holbrook (1753-1832) | |||
| Margaret Holbrook (1755-1805) | |||
| Comfort Holbrook (1756-) | |||
| Thankful Holbrook (1760-1834) | |||
| Mary Holbrook (1762-1814) | 1 February 1762 Uxbridge, Worcester County, Massachusetts, United States | 27 December 1814 Mendon, Worcester County, Massachusetts, United States | John Benson (1749-1818) |
| Stephen Holbrook (1764-1830) | |||
| Abigail Holbrook (1765-) | |||
| Henry Holbrook (1768-1868) | |||
| Lucy Holbrook (1770-1843) | |||
Siblings
| Name | Birth | Death | Joined with |
| John Holbrook (1717-1773) | 1 August 1717 Mendon, Worcester County, Massachusetts, United States | 23 November 1773 Uxbridge, Worcester County, Massachusetts, United States | Zilpah Thayer (1722-bef1762) |