Thomas Osborn was apprenticed to a Meadville printer and earned enough money to attend the preparatory department of Allegheny College (1855 - 57). He also had a few months of legal study in the office of Judge Derickson of Meadville in 1856.
In 1857 Thomas traveled westward to Pontiac, Michigan, where his career was officially launched by his admission to the bar just after his twenty-first birthday. In November of the same year he turned westward again and settled in Kansas. He first found work as a compositor in the office of the Kansas Herald of Freedom in Lawrence, and as acting editor during the absence of the owner. In the spring of 1858 he opened a law office in Elwood and in the same year was elected attorney of Doniphan County. His winning personality, energy, and ability had by this time been demonstrated to such a degree that in 1859 he took his seat as senator from Doniphan County in the first legislature of the new state of Kansas. He was a Republican and a Free-Stater. In 1862 he was elected president pro tempore of the Senate, though one of its youngest members, and presided with conspicuous ability at the impeachment of Gov. Charles Robinson. In the same year, 1862, he was elected lieutenant-governor of Kansas. In 1864 President Lincoln appointed him United States marshal, but political differences caused his removal by President Johnson in 1867.
In the election of 1872 Osborn was made governor of Kansas, and the following year he began his two eventful terms in that office. Three major crises arose, each of which he met with characteristic ability. His efficient relief measures during the "Grasshopper Year" of 1874 earned him the admiration and gratitude of the people of Kansas. The threat of a serious Indian uprising on the southern border of the state was successfully overcome by moderate but determined action. The discovery in 1875 of misconduct in the use of funds by the state treasurer was followed by prompt measures which averted what might have become a serious financial crisis. Under his administration the settlement of Kansas made great progress and many new counties were organized.
In 1877, after having unsuccessfully campaigned for a seat in the United States Senate, Osborn was appointed minister to Chile by President Hayes. During his residence at Santiago, Chile became involved in war with Peru and Bolivia. Osborn's attempts to effect a peaceful settlement between the countries were appreciated but futile. With the help of Thomas Ogden Osborn, American minister to Argentina, however, he was instrumental in settling the long-standing Patagonian boundary dispute, for which he received the public thanks of the government of Chile.
In 1881 he was appointed minister to Brazil by President Garfield. While no sensational event marked his residence at Rio de Janeiro, the Brazilian government showed its appreciation of his four years of service by bestowing upon him the highest honor that could be given a foreigner, the Grand Cross of the Order of the Rose. Returning to Kansas Osborn resumed his business and political interests. In 1888 he headed the Kansas delegation at the Republican National Convention. The same year he was elected state senator from Shawnee County and held office for two terms. He engaged in extensive business activities, including banking, real-estate, mining, investments, and railroads. He was a director of the Atchison, Topeka & Santa Fé Railroad from 1894 until his death. Osborn died suddenly in 1898, while on a visit to his old home in Meadville.
Thomas Osborn was a member of the State Senate from Doniphan County (1859) and from Shawnee County (1888). He also was a member of the Kansas delegation at the Republican National Convention (1888).
In 1870 Thomas Osborn married Julia Delahay, daughter of Judge Mark W. Delahay of Leavenworth, Kansas. They had one son.