Kinsella attended the common schools in New York.
After learning the printer's trade, Kinsella worked on the Cambridge Post, a weekly Whig newspaper in western New York. The publisher gave the lad free access to his library, and Kinsella read and studied eagerly to complete his grammar-school education. Editorial work attracted him and in addition to his compositor's duties he attempted articles for the paper. When the death of Henry Clay occurred, the editor being absent, Kinsella wrote an editorial on the statesman, and was much elated by the commendation it received.
Leaving the Cambridge Post, Kinsella in 1854 went South to familiarize himself with conditions there. Returning North in 1858, he obtained employment as a typesetter on the Brooklyn Daily Eagle. From the first, however, he contributed material to the paper. His ambition and ability attracted the attention of the proprietor, Isaac Van Anden, who promoted him to the position of law reporter. The Civil War found the Eagle under the editorship of Henry McCloskey, whose sympathies were with the South. So pronounced were his editorials that the government took notice of their treasonable character. McCloskey was forced to resign and Van Anden appointed Kinsella editor, September 7, 1861. That position he held, with short interruptions, until his death. He was a clear, forcible, and effective writer, and he supervised all departments of the paper, at the same time giving proper independence to those who won his confidence.
In 1865, the paper actively supported President Johnson, and in 1866 the President gave Kinsella a recess appointment as postmaster of Brooklyn, in which capacity he served for several months. The Senate, however, failed to confirm the appointment and he was displaced May 1, 1867. In 1868 he became a member of the Brooklyn board of education. In this connection his name is identified with two reforms: first, open bidding for supplies; and second, free opportunity and equal pay for women in the schools. A year later, he was appointed one of the three commissioners for the newly organized water and sewerage board. Finding that his duties consisted largely of "peddling out jobs" at the insistence of politicians, he resigned after a few months.
In 1870, he was elected to Congress for the 2nd District as a Democrat. In the 1880 campaign Kinsella, in the Eagle, was the first to propose General Hancock as a candidate. After election, convinced that the Kings County Democratic organization had not given Hancock wholehearted support, he opened up a bitter contest with "Boss" McLaughlin.
His health failed in 1883 and he traveled abroad, returning in the autumn apparently much improved. In December, however, he broke down again, and from this attack he never rallied. After three months' illness, he died at his home in Brooklyn, a splendid example of the immigrant Irish boy rising to wealth and honored position in the country of his adoption. With only a fair education as a foundation, his eagerness for learning, his industry, his honesty, and sincerity of purpose gained him a multitude of friends.
Thomas Kinsella was a member of the Democratic Party.
Kinsella was divorced from his first wife, by whom he had four daughters and later he married Emiline Van Siclen, the divorced wife of Thomas W. Field.