Allegany County was created by the state legislature on April 7, 1806, when Genesee County, New York, was partitioned to set aside some to the new county. The first County Seat was established at Angelica, New York, where it remained for half a century. It was later moved to Belmont, a village located along the Genesee River.New York. Laws of New York; 1806; 29th Session; Chapter 162; Section 1; Page 605. On March 11, 1808, the borders were adjusted so that of Steuben County passed to Allegany County,New York. Laws of New York; 1808; 31st Session, Chapter 38; Page 263. and of Allegany County passed to Genesee County.New York. Laws of New York; 1808; 31st Session; Chapter 40; Sections 1—2; Page 266. This established the current border between Genesee and Steuben counties, and reduced the size of Allegany County to . On June 12, 1812, the legislature authorized the attachment of Cattaraugus County, New York, to Allegany County for administration reasons, but for practical reasons this action did not take place at that time.New York. Laws of New York; 1812; 35th Session; Chapter 38; Page 263 . However, on April 13, 1814, the eastern half of Cattaraugus County was so attached and administered from Belmont.New York. Laws of New York; 1814; 37th Session; Chapter 123; Page 146. This attachment was ended on March 28, 1817.New York. Laws of New York; 1816; 40th Session; Chapter 115; Section 1; Page 107. With continued settlement through the mid-nineteenth century, the legislature periodically adjusted county borders as new counties were organized in western New York. On April 1, 1846, Allegany County lost to Wyoming County, reducing the size of Allegany County to , and establishing the current border between Allegany and Wyoming counties.New York. Laws of New York; 1846; 69th Session; Chapter 51; Page 53. On May 11, 1846, Allegany County lost to Livingston County, reducing the total to , and establishing the western portion of the current border with Livingston County.New York. Laws of New York; 1846; 69th Session; Chapter 197; Section 1; Page 235. On March 23, 1857, Allegany County lost another to Livingston County, passing the Ossian, New York, area to Livingston County, and establishing the current border between them.New York. Laws of New York; 1857; 80th Session; Chapter 166; Page 366.
Allegany County was created by the state legislature on April 7, 1806, when Genesee County, New York, was partitioned to set aside some to the new county. The first County Seat was established at Angelica, New York, where it remained for half a century. It was later moved to Belmont, a village located along the Genesee River.New York. Laws of New York; 1806; 29th Session; Chapter 162; Section 1; Page 605. On March 11, 1808, the borders were adjusted so that of Steuben County passed to Allegany CountyNew York. Laws of New York; 1808; 31st Session, Chapter 38; Page 263. and of Allegany County passed to Genesee County.New York. Laws of New York; 1808; 31st Session; Chapter 40; Sections 1—2; Page 266. This established the current border between Genesee and Steuben counties, and reduced the size of Allegany County to . On June 12, 1812, the legislature authorized the attachment of Cattaraugus County, New York, to Allegany County for administration reasons, but for practical reasons this action did not take place at that time.New York. Laws of New York; 1812; 35th Session; Chapter 38; Page 263 . However, on April 13, 1814, the eastern half of Cattaraugus County was so attached and administered from Belmont.New York. Laws of New York; 1814; 37th Session; Chapter 123; Page 146. This attachment was ended on March 28, 1817.New York. Laws of New York; 1816; 40th Session; Chapter 115; Section 1; Page 107. With continued settlement through the mid-nineteenth century, the legislature periodically adjusted county borders as new counties were organized in western New York. On April 1, 1846, Allegany County lost to Wyoming County, reducing the size of Allegany County to , and establishing the current border between Allegany and Wyoming counties.New York. Laws of New York; 1846; 69th Session; Chapter 51; Page 53. On May 11, 1846, Allegany County lost to Livingston County, reducing the total to , and establishing the western portion of the current border with Livingston County.New York. Laws of New York; 1846; 69th Session; Chapter 197; Section 1; Page 235. On March 23, 1857, Allegany County lost another to Livingston County, passing the Ossian, New York, area to Livingston County, and establishing the current border between them.New York. Laws of New York; 1857; 80th Session; Chapter 166; Page 366.