Henry became father to younger Henry Shelley. This younger Henry had at least three sons. The youngest of them Richard Shelley was later married to Joan Fuste, daughter of John Fuste from Itchingfield, near Horsham, West Sussex. Their grandson John Shelley of Fen Place, Turners Hill, West Sussex, was married himself to Helen Bysshe, daughter of Roger Bysshe. Their son Timothy Shelley of Fen Place (born c. 1700) married widow Johanna Plum from New York City. Timothy and Johanna were the great-grandparents of Percy.
Percy was born to Sir Timothy Shelley (7 September 1753 – 24 April 1844) and his wife Elizabeth Pilfold following their marriage in October 1791. His father was son and heir to Sir Bysshe Shelley, 1st Baronet of Castle Goring (21 June 1731 – 6 January 1815) by his wife Mary Catherine Michell (d. 7 November 1760). His mother was daughter of Charles Pilfold of Effingham. Through his paternal grandmother, Percy was a great-grandson to Reverend Theobald Michell of Horsham. Through his maternal lineage, he was a cousin of Thomas Medwin — a childhood friend and Shelley's biographer
Percy was the eldest of six children. His younger siblings were:
- John Shelley of Avington House (15 March 1806 – 11 November 1866; married on 24 March 1827 Elizabeth Bowen (d. 28 November 1889));
- Mary Shelley (NB. not to be confused with his wife);
- Elizabeth Shelley (d. 1831);
- Hellen Shelley (d. 10 May 1885);
- Margaret Shelley (d. 9 July 1887).
Shelley's uncle, brother to his mother Elizabeth Pilfold, was Captain John Pilfold, a famous Naval Commander who served under Admiral Nelson during the Battle of Trafalgar.
Three children survived Shelley: Ianthe and Charles, his daughter and son by Harriet; and Percy Florence Shelley, his son by Mary. Charles, who suffered from tuberculosis, died in 1826 after being struck by lightning during a rainstorm. Percy Florence, who eventually inherited the baronetcy in 1844, died without children. The only lineal descendants of the poet are therefore the children of Ianthe.
Ianthe Eliza Shelley was married in 1837 to Edward Jeffries Esdaile of Cothelstone Manor. The marriage resulted in the birth of one daughter, Una Deane Esdaile (Thurston), who married Campbell Carlston Thurston and had two children, Mildred and Alan. Several members of the Scarlett family were born at Percy Florence's seaside home "Boscombe Manor" in Bournemouth. The 1891 census shows Lady Shelley living at Boscombe Manor with several great nephews.