In 1894, President Grover Cleveland appointed White as an Associate Justice of the Supreme Court of the United States. In 1910, President William Howard Taft elevated him to the position of Chief Justice. The appointment surprised many contemporaries, as Taft was a member of the Republican Party . White served as Chief Justice until his death in 1921, when he was succeeded by Taft.

White's studies at Georgetown were interrupted by the Civil War. It has been suggested[by whom?] that he returned to Bayou Lafourche, where he purportedly enlisted as an infantryman in the Confederate States Army under General Richard Taylor and eventually attained the rank of lieutenant. This is questionable[by whom?], as his widowed mother had remarried and was living with the rest of the family in New Orleans at the time. When he returned to Louisiana, it was probably to his primary home in New Orleans.[citation needed]

An apocryphal account states that White was almost captured by General Godfrey Weitzel's Union army when they invaded Bayou Lafourche in October 1862, but that he evaded capture by hiding beneath hay in a barn. It is possible that White enlisted in the Lafourche militia, as its muster rolls are not complete. There is no documentation, however, that White served in any Confederate volunteer unit or militia unit engaged in campaigns in the Lafourche area.

Another account suggests that he was assigned as an aide to Confederate General W. N. R. Beall and accompanied him to Port Hudson, Louisiana, which had a garrison of 18,000 Confederate soldiers, but a numerically superior Union force surrounded it. After a siege lasting from May 21 to July 8, 1863 (the longest siege in North American history), the Confederate forces unconditionally surrendered after learning of the fall of Vicksburg to the Union. White's presence at Port Hudson, when he was 18 years old, is supported by a secondhand account of a postwar dinner conversation he had with Senator Knute Nelson of Minnesota, a Union veteran of Port Hudson, and another recounted by Admiral George Dewey (then a Federal naval officer at Port Hudson), in both of which White referred to being part of the besieged forces. But, White's name does not appear on any list of prisoners captured at Port Hudson. According to another account of questionable reliability, White was supposedly sent to a Mississippi prisoner of war camp. (As practically all Confederate soldiers of enlisted rank of the Port Hudson garrison were paroled, and officers sent to prison in New Orleans and later to Johnson's Island, Ohio, this account is likely not true.) When White was paroled, he supposedly returned to the family plantation, to find it abandoned, the canefields barren, and the place nearly empty of most former slaves.

The only "hard" evidence of White's Confederate service consists of an account of his capture on March 12, 1865 in an action in Morganza in Pointe Coupee Parish, which is contained in the Official Records of the American Civil War, and his service records in the National Archives, documenting his subsequent imprisonment in New Orleans and parole in April 1865. These records confirm his service as a lieutenant in Captain W. B. Barrow's company of a Louisiana cavalry regiment, for all practical purposes a loosely organized band of irregulars or "scouts" (guerrillas). One organizing officer of this regiment, which was sometimes called "Barrow's Regiment" or the "9th Louisiana Cavalry Regiment," was Major Robert Pruyn. Pruyn (a postwar mayor of Baton Rouge, Louisiana) served as courier relaying messages from Port Hudson's commander, General Franklin Gardner, to General Joseph E. Johnston, crossing the Union siege lines by swimming the Mississippi. Pruyn escaped from Port Hudson prior to its surrender in the same manner. According to another account, after White was paroled in April 1865 and following the surrender of the western Confederate forces, he ended his military career by walking (his clothing in rags) to a comrade's family home in Livonia in Pointe Coupee Parish.

White's Civil War service was taken as a matter of common knowledge at the time of his initial nomination to the United States Supreme Court, and the Confederate Veteran periodical, published for the United Confederate Veterans, congratulated him upon his confirmation. White was one of three ex-Confederate soldiers to serve on the Supreme Court. The others were Associate Justices Lucius Quintus Cincinnatus Lamar (II) of Mississippi and Horace Harmon Lurton of Tennessee. The Court's other ex-Confederate, Associate Justice Howell Edmunds Jackson, had held a civil position under the Confederate government.

White was a member of the Ku Klux Klan in the aftermath of the death of the Confederacy. Later, while on the Supreme Court, he approved of the film The Birth of a Nation, which helped reignite the Klan in the 1920s.[3]

In 1877, White served on the Reception Committee of the Knights of Momus in New Orleans, whose widely condemned extreme political satire of Reconstruction and its aftermath earned even the vilification of the Krewe of Rex, according to The New Orleans newspaper The Republican of February 14, 1877.