Hugh Birnie was one of eleven children born to Clotworthy Birnie I (1765–1845), a tenant farmer on the Upton family estate in County Antrim, Northern Ireland, and Margaret Scott Birnie (1717-1803), who came from a prominent Presbyterian middle-class background. Margaret's brother, Dr. Upton Scott (1724-1814), was a notable figure in colonial Maryland, serving as the personal physician to Horatio Sharpe (1718-1790), the last Royal Governor of Maryland. This prestigious role enabled Dr. Scott to establish a thriving medical practice and hold the "Court Physician" title in the capital. After completing his medical training in Ireland and Scotland, Dr. Scott joined the British Army as a surgeon's mate before accompanying Governor Sharpe to Maryland. Once there, Dr. Scott married Elizabeth Ross (1730-1819) and integrated into the colony's political and social elite, eventually shifting from his medical career to administrative positions, including serving as the final clerk of the proprietary government before the Revolution.

Despite Dr. Scott's success and connection to the Birnie family, he disapproved of his sister's marriage to Clotworthy Birnie I. Nonetheless, he occasionally supported his sister and her children, providing education and opportunities for several of his nephews, including arranging a medical education in Britain for John Birnie and securing merchant berths for Francis Upton Birnie. However, both sons met tragic ends—John died during his medical training, and Francis perished at sea during the Revolutionary War. Young Clotworthy II eventually emigrated to America, settling in the Taneytown area in 1810.

Despite Dr. Scott's disapproval, Hugh Birnie emigrated to Philadelphia in 1763. His uncle was critical of Hugh's decision, expressing skepticism about his abilities. Dr. Scott's letter to Hugh’s father detailed his attempt to help Hugh find respectable employment. However, he noted Hugh's reluctance to pursue opportunities like going to sea or working in a counting house. Ultimately, Hugh found work as a servant to an innkeeper. Over time, he learned the cooper's trade and settled in Baltimore.

Birnie's correspondence with his uncle was infrequent and strained. He later explained that political differences during the Revolution, where Birnie supported the Whigs while his uncle aligned with the Tories, further distanced them. Birnie also mentioned his cousin, Captain Hugh Scott, who had similarly fallen out of favor with both Birnie and the Taneytown community for his pro-Tory sentiments. After the war, Captain Scott sold his Frederick County lands and moved to Virginia before disappearing from records after 1798 in the Pittsburgh area.

In 1783, as peace returned to the American colonies, Hugh Birnie wrote to his mother in Ireland, expressing relief that the war had finally ended. With the conflict over, communication could resume, and the promise of prosperity in America seemed within reach. He reflected on the difficulties Irish immigrants like himself faced during the war, citing fluctuating business, high prices, and the constant disruption caused by the British. Despite these challenges, Birnie was optimistic about the future, particularly the potential for trade in the newly established republic. He shared that his cooperage business in Baltimore was thriving, though he remained unmarried.

The letter also touches on Birnie's strained relationships with his uncle, Dr. Upton Scott, and his cousin, Captain Hugh Scott. Despite their differences, Birnie expressed his continued respect for his uncle and his duty to him, though Dr. Scott had been distant and unresponsive to Birnie's letters. Birnie believed Captain Scott's coolness toward him stemmed from their opposing political allegiances during the Revolution. Birnie provided updates on other family members, inquired about relatives in Ireland, and expressed concern for his younger brother, Clotworthy II.

He advised his mother to ensure that Clotworthy II received a proper education and was placed in a suitable trade. Birnie believed that business success depended on education and connections, and he hoped his brother would be guided by someone experienced in commerce.

Despite the optimistic tone of his letter, hardship marked Birnie's later life. After remaining in the Baltimore area for some time, a permit from 1791 allowed Birnie and his apprentice to travel through the state for business. By 1798, Birnie had moved to Pittsburgh, where he worked in flour mills. However, around 1805, he suffered a severe leg injury in an accident, which left him unable to work. He settled in Freeport, Pennsylvania, facing financial difficulties because of title issues with his small land holdings.

He maintained some contact with Captain—now Dr., having returned to the medical profession—Hugh Scott, Dr. Upton Scott, and Clotworthy Birnie, II, his younger brother who had recently immigrated. The latter two occasionally sent him money. Eventually, after Clotworthy inherited much of Dr. Upton Scott's estate and established himself in style at "Thorndale" on Big Pipe Creek outside of Taneytown, he brought his elder brother, who was in poor health and financial ruin, to live out his final days in comfort by the family hearth. A gravestone in the family plot at Piney Creek Presbyterian Church near Taneytown marks his passing: "In memory of Hugh Birnie, a Native of Ireland, who died August 21st, 1822. Aged 76 years."

Hugh Birnie's letters provide a rare and valuable insight into the life of an ordinary artisan during and after the Revolutionary War. His reflections on family, politics, and business offer a personal perspective on the challenges and opportunities many foreign-born colonists face in America. While Birnie's optimism for the future of the new republic shines through, his later life illustrates the enduring struggles of many who sought a better life in post-Revolutionary America.