Gregory was born on November 6, 1861, in Crawfordsville, Arkansas, the son of Capt. Francis Robert and Mary Cornelia (Watt) Gregory, one of two children, the elder having died before Thomas was born. His father was a native of Mecklenburg County, Virginia, a physician who enlisted in the Confederate army and was killed during the Civil War. Thomas was reared in the home of his maternal grandfather, Maj. Thomas Watt, a Mississippi planter.
Gregory graduated from Southwestern Presbyterian University, then located at Clarksville, Tennessee, in 1883, the first student to complete the college course in two years. In 1883-1884 he was a law student at the University of Virginia, where he was a classmate of James C. McReynolds, his predecessor in the attorney-general's office. Gregory distinguished himself at Virginia by winning the Jefferson debater's medal. In 1885 he was graduated from the law school of the University of Texas with the degree of LL. B.
Gregory opened a law office in Austin, Texas after graduation. After practising alone for five years he formed a partnership with Robert L. Batts. He served as assistant city attorney of Austin for the years 1891-1894 and was offered but declined appointments as assistant attorney-general of Texas in 1892 and a state judgeship in 1896. The most important case handled by the firm of Gregory & Batts was that of the State of Texas against the Waters Pierce Oil Company, in which the company was charged with violating the Texas anti-trust laws. Gregory was a delegate to the Democratic national conventions of 1904 and 1912. He worked with Col. Edward M. House to secure Woodrow Wilson's nomination for president, handling the promotional work in Texas and acting as one of House's lieutenants in the convention at Baltimore. The result of the combined efforts of the two men was that the Texas convention elected a delegation instructed to vote for Wilson with no second choice. This group played a most important part in his nomination, as is evinced by the fact that Tammany Hall offered to support Senator Charles A. Culberson of Texas if the Texas delegation would repudiate Wilson. Shortly after Wilson became president, Gregory was appointed special assistant to the United States attorney-general to bring action against the New York, New Haven & Hartford Railroad on the charge that it was monopolizing the transportation facilities of New England. Gregory represented the government in these negotiations, the outcome of which was that the road gave up control of the Boston & Maine, and the Boston & Albany gave up its interests in trolley lines and coastwise shipping. Gregory's success in this work, together with the influence of Colonel House, led to his appointment as attorney-general in 1914 when McReynolds was appointed to the Supreme Court. The period during which Gregory was head of the Department of Justice was one of the most important in its history. After the outbreak of the First World War and before the United States entered it, the principal activity of the department was that of circumventing the work of agents of foreign governments and preventing or suppressing violations of American neutrality. This work necessitated the creation of a war emergency division within the department, and the Federal Bureau of Investigation grew to five times its normal size. With the entry of the United States into the war the enforcement of the espionage, sedition, sabotage, and trading-with-enemy acts was added to the department's duties, and the passage of the Selective Service Act increased still further its labors. Gregory's reports reveal that it caused the arrest of 6, 300 spies and conspirators, 2, 300 of whom were detained in army detention camps, the remainder on parole; brought 220, 747 actions against men who failed to comply with the Selective Service Act; and uncovered the activities of a ring which was securing government munition and supply contracts through the payment of contingent fees. It also organized and superintended the operations of the American Protective League, a volunteer secret service, and assisted the work of the alien property custodian. In addition to these war activities Gregory started several anti-trust suits, including those against the International Harvester Company and the anthracite coal operators. He also secured reforms in the administration of federal prisons. When Justice Hughes resigned from the Supreme Court in 1916, President Wilson offered the place to Gregory, who declined it because his hearing was impaired and also on the ground that he did not like the confining life that the position necessitated. He is said to have worked with Colonel House for the appointment of liberals to the Supreme Court, and it is also asserted that he urged Wilson to take with him to the Peace Conference. He resigned from the cabinet March 4, 1919, and, after a trip to Europe, where he was an adviser to the Peace Conference, practised law in Washington as a member of the firm of Gregory & Todd. After a few years, however, he moved to Houston, Texas, where he led a retired life, varied by a few law cases and much work on behalf of the students of the University of Texas. Gregory died of pneumonia on February 26, 1933, in New York, where he had gone to confer with the newly elected president, Franklin D. Roosevelt.
Gregory married Julia Nalle, daughter of Capt. Joseph Nalle of Austin, Texas on February 22, 1893; they had four children, two sons and two daughters.
