Serving during the rest of the war as a government stenographer, he was present at the death of Abraham Lincoln and took notes that are the most comprehensive record of the events of the President's assassination.

He later served as the United States Commissioner of Pensions, and helped reorganize and incorporate the American Red Cross.

[8] Paroled after 10 days, he spent several weeks recovering in Fairfax Seminary Hospital before being sent home to New York.

[9] In October 1864, Tanner won an appointment as a clerk and stenographer in the Ordnance Department in Washington, D.C.[9][10] On April 14, 1865, he was summoned to the Petersen House where Abraham Lincoln lay dying from an assassin's bullet.

[9][13] He took a job as a clerk of a committee in the state legislature,[3] studied law with Judge William C. Lamont, and was admitted to the bar in 1869.

[7] The election of a Democratic city administration in 1886 forced him from office, and he became a popular public speaker on the chataqua circuit.

[3][16] Tanner was frequently called on to lobby Congress on behalf of veterans and made many speeches in favor of Benjamin Harrison's presidential candidacy.

[14] His willingness to hire disabled veterans rather than party hacks[18] and his desire to "treat the boys liberally"[19] and loosen rules so that veterans could more easily qualify for pensions[20] led to an investigation by Secretary of the Interior John Willock Noble, Tanner's superior.

His fame as a disabled veteran and witness to the Lincoln assassination made him popular among GAR members, and in 1876 they elected him Commander of the New York state organization.

[3][13][25][24] Many attempts had been made in the previous decade to create an old soldiers' home in the state, but none of these efforts bore any fruit.

He enlisted the help of the Reverend Henry Ward Beecher, a noted abolitionist and firebrand preacher, and held an organizing meeting in Brooklyn which raised $13,000 ($371,963).

[3][24] In 1912, the United Daughters of the Confederacy invited him to the ceremony for the laying of the cornerstone for the Confederate Memorial in Arlington National Cemetery.

Such was his fame that he was asked on the spot to deliver short remarks to the assembled Confederate veterans (even though he was not scheduled to speak).

Tanner worked assiduously to win a Congressional charter for the reorganized American Red Cross, which was granted in 1904.