Pelham's many illuminating letters, especially to his half-brother and fellow painter John Singleton Copley, provide an important contemporary perspective of the events of the American Revolution.
A small tobacco shop run by his mother provided support for the family until Copley brought prosperity to them all through his portrait painting.
A likeness of Pelham, then aged fifteen or sixteen, is featured in A Boy with a Flying Squirrel, a painting that was exhibited in London in 1766 and brought Copley his first fame abroad.
In the winter of 1775, while making a journey on horseback to Philadelphia, a mob attacked him in Springfield, Massachusetts, as one of "a damn'd pack of Torys."
Along with other Loyalists, Pelham left Boston for London in August 1776, just following the issuance of the Declaration of Independence by the Second Continental Congress in Philadelphia, which formalized and escalated the American Revolutionary War.