The 87th New York State Legislature, consisting of the New York State Senate and the New York State Assembly, met from January 5 to April 23, 1864, during the second year of Horatio Seymour's second tenure as Governor of New York, in Albany.

Under the provisions of the New York Constitution of 1846, 32 Senators and 128 assemblymen were elected in single-seat districts; senators for a two-year term, assemblymen for a one-year term. The senatorial districts were made up of entire counties, except New York County (four districts) and Kings County (two districts). The Assembly districts were made up of entire towns, or city wards, forming a contiguous area, all within the same county.

At this time there were two major political parties: the Republican Party and the Democratic Party. The Democrats split over the civil war issue. The "War Democrats" and the Republicans formed a coalition known as the "Republican Union," and supported President Abraham Lincoln and the Union Army's war effort; the rump Democratic Party opposed the war, favoring a compromise with the South, and became known as "Peace Democrats" or "Copperheads."

The New York state election, 1863 was held on November 3. All eight statewide elective offices up for election were carried by the Republican Union. The approximate party strength at this election, as expressed by the vote for Secretary of State, was: Republican Union 314,000 and Democrats 285,000.

The Legislature met for the regular session at the Old State Capitol in Albany on January 5, 1864; and adjourned on April 23.

Thomas G. Alvord (R) was elected again Speaker with 77 votes against 42 for Jacob L. Smith (D). Joseph B. Cushman (R) was re-elected Clerk of the Assembly with 78 votes against 44 for John C. Jacobs (D).

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