Miles' great-uncle was Simon Knubley, described as a 'large-scale attorney and slave-owner in his own right in Jamaica' by the Legacies of British Slavery project.¹

Simon was the sole awardee for his estate in Jamaica, receiving £1350 12s 10d in compensation for 67 enslaved people.² Until 1832, he also owned another estate in Jamaica, as well as the 77 enslaved people registered on it in that year.³ He also served as the attorney for absentee owners across 12 other estates on Jamaica, appearing as such in slave registers between 1817 and 1832.¹

In his will he requested his estate in Jamaica be sold and the proceeds passed to the second son of his nephew Edward Carr Knubley, identified in his will as '____' Knubley.³ Edward Carr Knubley was Miles' father.⁴ Miles Ponsonby was thus a primary beneficiary of Simon having owned land and enslaved people in Jamaica (prior to his death there in 1836). 

In 1859 administration over Simon's estate was granted to Ann Knubley (Miles' widow), as the guardian of Edward Ponsonby Knubley, the young son of Ann and Miles, and the residuary heir of Simon Knubley's estate.¹ Miles' son Edward matriculated at Magdalene College, Cambridge in 1869.⁵