Albany County was an enormous county, including the northern part of New York State as well as all of the present State of Vermont and, in theory, extending westward to the Pacific Ocean.

The county was named for William Tryon, British colonial governor of New York.

In the years prior to 1776, most of the Loyalists in Tryon County fled to Canada.

In 1784, following the peace treaty that ended the American Revolutionary War, the name of Tryon County was changed to Montgomery County to honor the general, Richard Montgomery, who had captured several places in Canada and died attempting to capture the city of Quebec, replacing the name of Tryon.

During the late 1780s, New York State governor De Witt Clinton issued land grants to military officers, doctors and other veterans of the American Revolutionary War, which brought many patriot veterans to settle in the area.

The county is within a region called the Southern Tier of New York State.

The highest point is an approximately 3,520-foot (1,070 m) summit of Bearpen Mountain along the Greene County line.

The State University of New York at Delhi is located in Delaware County.

DCMO BOCES (Delaware-Chenango-Madison-Otsego; Board of Cooperative Educational Services) – Robert W. Harrold Campus is located in Sidney Center.

The county has voted for two Democratic presidential candidates in its history, Bill Clinton in 1996 and Lyndon B. Johnson in 1964.

The last Democrat to win the majority of votes on the gubernatorial level was Andrew Cuomo in 2010.