William was the son of Anna Matilda McNeill and George Washington Whistler; he was James's younger brother. He was married twice: (1) Ida Bayard King (d. 1863), in Brooklyn, NY; (2) Helen Euphrosyne Ionides (1849-1917), in London.
From 1850-52, William McNeill Whistler attended Christ Church School, Pomfret, and St James College, Williamsport, MD. In 1853, he entered Columbia College, NY but never graduated. In March 1855, he resumed his studies at Trinity College, Hartford, CT, where he enrolled as a pre-medical student.
In 1857, he became apprentice to Dr James Darrach of Philadelphia, and from 1858 to 1860, he studied Medicine at the Pennsylvania Medical School.
In 1861, he joined the Confederate army as an Assistant Surgeon and served in various Richmond locations including Libby Prison, and Drewry's Bluff on the James River near Petersburg. In March /April 1863 he was at Camp Jackson, near Richmond. He served with Orr's (S. C.) Rifles, Colonel G. McD. Miller, in Brigadier General Samuel McGowan's S. Carolina brigade, General Cadmus M. Wilcox's division, in the last Virginia campaign. The regiment saw action at: Spotsylvania, in which he was recorded for bravery; Jericho Ford on the North Ann; Deep Bottom on the James River; Fussel's Mills; Remas Station on the railway from Richmond to Weldon, NC; Jones' Farm.
In 1865, he went to Britain where he settled for the rest of his life. He was admitted a Member of the College of Surgeons of England and to the Royal College of Physicians in 1876. He practiced at several London hospitals including St George's Hospital for Diseases of the Throat and London Throat Hospital where he was Senior Physician until his death.
Whistler painted several portraits of his brother including Portrait of Dr William McNeill Whistler y123; Portrait of Dr Whistler, No. 2 c111; Portrait studies of the artist, his brother Dr Whistler, and others m1016.
[Robert R. Hemphill], 'William McNeill Whistler, MD.,' Confederate Veteran, Nashville, TN, 1900, vol. 8, pp. 282-83; Edward Law, Obituary: William MacNeill [sic] Whistler, M.D., Senior Physician, London Throat Hospital, London: British Medical Association, 1900; The Correspondence of James McNeill Whistler, 1855-1903, edited by Margaret F. MacDonald, Patricia de Montfort and Nigel Thorp; including The Correspondence of Anna McNeill Whistler, 1855-1880, edited by Georgia Toutziari. Online edition, University of Glasgow.