Dimmock was engaged in newspaper work for thirty years. He was in management of the Alton Democrat from April 1853 to November 1864 and then became the editor in January 1867. He was a "staunch supporter of the Union cause and rendered much service to Abraham Lincoln by making speeches and by his writings in 1860" and after. His last regular editorial connection was with the Missouri Republican, where he was for a time the city editor and for which he wrote editorials, book reviews, critical articles and other matter from 1869 to 1888. He then devoted himself to independent literary work and lecturing. An obituary in the St. Louis Globe-Democrat said of him: "He was a pleasing and scholarly writer and delighted in historical subjects, especially those about which clustered some problem or mystery. He could always write something new about Lincoln because he knew him personally." In 1891–1894, he authored a series of articles in the Post-Dispatch headed "Things Wise and Otherwise." He also wrote articles on "Dean Swift, with his two loves, 'Stella' and 'Vanessa.'"
In 1864, Dimmock "reclaimed from oblivion" the Alton, Illinois, grave of free-press martyr Elijah Parish Lovejoy, who was shot and killed by a pro-slavery mob in 1837. Dimmock "succeeded in establishing the location of the grave... in a roadway where vehicles were passing over it.... Mr. Dimmock had the bones disinterred and... laid in a new grave where they would be free from trespass." He was principal orator at the dedication of a later monument erected to Lovejoy's memory.
Dimmock was born as Thomas Dimmick on May 22, 1829, in Falmouth, Massachusetts, the son of Elijah L. and Sarah Dimmick. He removed with his parents in the early 1860s to Alton, Illinois, where he was educated. He and his first wife, Maria Tilton, had a daughter, Theodosia Burr, whom they named after Theodosia Burr Alston, the daughter of Aaron Burr, whose ship on which she was sailing was lost near Cape Hatteras in a storm in 1813. Maria died in 1860. His second wife was Caroline Garnier; they were married in St. Louis on February 6, 1873. Dimmock died November 18, 1909, in St. John's Hospital, St. Louis. He had been comatose for three years. A funeral service was held in the home of his daughter, Mrs. Jacob Sead, on Second Street, "the old Dimmock homestead." George R. Dodson, pastor of the Church of the Unity, officiated. Dimmock was interred in the Alton City Cemetery.
