[3] The county is named in honor of the family of Kiliaen van Rensselaer, the original Dutch owner of the land in the area.

The area that is now Rensselaer County was inhabited by the Algonquian-speaking Mohican Indian tribe at the time of European encounter.

Kiliaen van Rensselaer, a Dutch jeweler and merchant, purchased the area in 1630 and incorporated it in his patroonship Rensselaerswyck.

The terrain runs from level and flat near the Hudson and then rises into the Rensselaer Plateau around Poestenkill and Sand Lake, then to the Taconic Mountains along the Massachusetts state line.

The lowest point is 62 feet (19 m) above sea level at the Hudson River's southernmost extent in the county.

Depending on precise location within the county, road travel distance to New York City ranges between 132 and 178 miles (212 and 286 km).

22.3% were of Irish, 14.7% Italian, 12.8% German, 7.5% English, 6.2% French, 5.3% American and 2.3% Puerto Rican ancestry according to Census 2010.

The Board of Supervisors form of government was terminated as a result of a class action lawsuit brought by Troy attorney Marvin I. Honig on behalf of his wife, Nedda, during March 1968.

Mr. Honig's motive in bringing the lawsuit was to punish the Rensselaer County Republican Party chairman and certain members of the Board of Supervisors for defaulting on an agreement with him.

The NY Supreme Court ruled in Honig's favor, and ordered the creation of a legislative body.

The first "Honig Plan" was drawn to favor the Democratic party, which had not had control of the county government in decades.

One notable candidate for Executive was Edward Pattison who was later elected to Congress, and whose son Mark served two terms as Mayor of Troy.

Legislative authority is vested in the County Legislature, which consists of 19 members representing 17 different communities, separated into six districts.

[20] The county is also home to Rensselaer Polytechnic Institute, the oldest operating technological college in America.