Although associated with the liberal and progressive reform movements, he became widely known as the leading national spokesman for the Tammany Society, the powerful Democratic Party political machine in New York.

He was a leading orator of the late 19th and early 20th century, compared favorably by historians to his contemporary political rival, William Jennings Bryan, and to Edmund Burke.

On his arrival, he remarked to his mother that he was "a good deal bewildered" by the Gilded Age contrasts in New York society between fabulous displays of wealth alongside abject poverty of the tenements and streets.

[7] He gained employment as a clerk at the A. T. Stewart & Company department store, though he never appeared for work, and secured a position as a tutor at St. Teresa's Academy, a private day school for girls on Rutgers Street.

[8] After briefly returning to Ireland to cover the unveiling of a monument to Daniel O'Connell in Dublin as a correspondent for the New York Herald, he declined a position at the foreign news desk and became principal of a public school in Tuckahoe, Westchester County.

[12] Cockran was a strong opponent of municipal ownership, though he criticized Brooklyn Union's president, James Jourdan, for managing the company "in a spirit of indifference to the public interest".

In 1887, Cockran defended Jacob Sharp, a railroad lobbyist accused of bribing the Board of Aldermen to secure a franchise to build and operate a streetcar line on Broadway.

In 1889, he handled the unsuccessful appeal of death row inmate William Kemmler, who became the first man executed by means of electric chair the next year.

[12] Beginning in 1916, he led a long legal battle to exonerate socialist activist Tom Mooney, who was convicted of the 1916 Preparedness Day bombing in San Francisco.

These speeches greatly enhanced his reputation as an orator, and he joined the national campaign in support of Winfield Scott Hancock for president, travelling to the American Midwest for the first time.

[16] For the remainder of his career, he was the leading spokesman for the Tammany position on national issues, though he avoided local and state political debates, and his standing as a member waned over time.

[18] Cleveland, who had been elected in 1882 with Tammany support, governed New York as a non-partisan reformist and distributed patronage positions with preference for merit over party loyalty, angering Kelly.

In 1904, Cockran won a special election return to the House in the empty seat of George B. McClellan Jr., who had resigned to become mayor of New York City.

In 1896, Cockran publicly broke with the Democratic Party, opposing the Free Silver platform of presidential candidate William Jennings Bryan.

Churchill wrote in the 1930s that Cockran was, "A pacifist, individualist, democrat, capitalist, and a 'Gold-bug'....He was equally opposed to socialists, inflationist, and Protectionists, and he resisted them on all occasions."

Churchill never became a pacifist but he did adopt all the rest of Cockran's stances during his own political career, and carefully read and reread his speeches for oratorical advice.