One genealogy has John's date of birth as June 1695 (source: A Brief Genealogy of the Usher Family of New England, by William Henry Whitmore, at Google Books). This genealogy also has his date of death as 1 May 1775.
Also see American Biography: a New Cyclopedia, Volume 5, By American Historical Company, at Google Books. This genealogy has John's parents as John Usher and his first wife Elizabeth Lidgett.
From Records of the First Church in Boston (at americanancestors.org): "John son of John Usher, baptised at his Father's house in Mystick, on Tuesday, Sept. 10, 1700, three brethren of our church, with several other christians, were present . . . Usher had returned from New Hampshire and was living in Mystic (i.e. Medford)." This would suggest that John (Jr.) was born about Sep 1700, since they practised infant baptism.
For 52 years he was the rector of St. Michael's Church in Bristol, RI.
Various genealogies of the Usher family have different parentage and grandparentage for John; see above notes and sources.
Abt 1700 - 1775 (74 years)