It depicts Copley's teenaged half-brother Henry Pelham with a pet flying squirrel, a creature commonly found in colonial American portraits as a symbol of the sitter's refinement.

[2] To test whether his art met English standards, Copley completed A Boy with a Flying Squirrel by early fall 1765, which was to be viewed by a London audience.

[7] Copley rendered a variety of colors and textures, including the red drapery in the background, the highly polished mahogany table, the boy's skin, the squirrel's fur, and the reflections in the glass of water.

[7] A Boy with a Flying Squirrel was completed while Copley was still based in Boston,[12] but it needed to be shipped across the Atlantic Ocean for a spring 1766 exhibition in London at the Society of Artists of Great Britain.

Bruce in turn gave the work to a Lord Cardross who took it to the studio of Britain's leading painter Sir Joshua Reynolds.

[3] Reynolds further tempered his enthusiasm for the painting by adding that Copley needed to receive proper training before his "Manner and Taste were corrupted or fixed by working in [his] little way at Boston".

[10] A Boy with a Flying Squirrel also garnered the attention of Benjamin West, who in a letter to Copley recognized "the great Honour the Picture has gaind you" but found the painting "to [sic] liney".

[3] The painting's success in England encouraged Copley to make more disciplined portraits in his late American career and strengthened his conviction to move to London, which he finally did in 1774.

[21] The Museum of Fine Arts, Boston, celebrated Copley's bicentennial with a February 1 to March 15, 1938 exhibition that included A Boy with a Flying Squirrel.

According to the art historian William Dunlap, the Scottish writer Allan Cunningham judged that "of all that [Copley] ever painted, nothing surpasses his Boy and Squirrel for fine depth and beauty of colour".

In 1983, the art historian Trevor Fairbrother drew parallels between the painting and "European devotional pictures" as well as the French painter Jean Siméon Chardin's scenes of boys playing with cards.

[28] The art historian Jennifer Roberts believed A Boy with a Flying Squirrel to be "the most consequential painting Copley had yet produced in America".

[30] In a broader sense, Roberts viewed the "triumph" of the painting's critical success as an "originary episode" in the history of American art.