Captain in the 7th Regiment of Foot in the American War and promoted to Major while there ca. 1776. Newmarsh was married to Mary Trapaud (d. 1791), with whom he had at least two children. One, Alexander Newmarsh, was a Captain in the 17th Regiment of Foot who died at Santo Domingo in 1796. The other, John Ludovez Newmarsh, lived until at least 1825 and was party to a prolonged series of court disputes concerning the property of his late father-in-law (d. 1796) and his second wife (d. 1798).
Boswell and Dr Johnson met Captain Newmarsh, as he then was, together with his wife and father-in-law at Fort Augustus on August 30, 1773. Boswell did not write anything specifically about Newmarsh, but remarked in his journal that they "were all most obliging and polite."