Matthew Vassar, aged 18, returned to Poughkeepsie with $150, "the nett earnings of his Foreign Service." He took over the bookkeeping for his father's brewery business. 

Matthew Vassar bought from Peter Cooper for $300 the patent-right for machinery for shearing cloth. This was the inventor and industrialist’s first bill of sale. 

James Vassar's brewery was destroyed by fire. Matthew Vassar's elder brother, John Guy, who had joined their father in the brewery business, was killed by fumes while inspecting the debris.

Devastated, James Vassar retired to farm life. His son Matthew started brewing independently, in a dye house owned by his brother-in-law, George Booth, a textile manufacturer.  The young Matthew also assumed responsibility for his sister-in-law and her two small sons, John Guy Vassar, Jr. and Matthew Vassar, Jr.

Outbreak of War of 1812. The United States declared war on Great Britain on the grounds of impressment of American seamen, violation of U.S. neutral rights and territorial waters, blockade of U.S. ports and refusal to revoke Orders in Council. 

Matthew Vassar rented a basement room in the County Court House and opened the first oyster saloon or restaurant in the village of Poughkeepsie, peddling beer by day and tending the oyster saloon at night. 

Matthew Vassar married Catharine Valentine from Fishkill. "In the following summer 1812 [sic] began the world,—that is the buisness world for myself by getting married and begining house-keeping, Renting part of a tenement at $40 pr year, and was severely rebuked by my Father for my extravagence —$25 pr year was as much as he thot I ought to pay...."     Elizabeth Hazelton Haight, ed., Autobiography and Letters of Matthew Vassar

Matthew Vassar entered into partnership with Thomas Purser, the existing co-partnership of M. Vassar & Co. having been dissolved in July. The brewery on Vassar and Bridge Streets was built. Purser's interest was sold to J.M. and N. Conklin, Jr. on June 10, 1815. 

British forces entered Washington D. C., burning and looting the White House, Capitol, Treasury and War Department and forcing President James Madison and his government to flee to Maryland.

The subsequent American response and the defense of Fort McHenry in the Battle of Baltimore, September 13-14, inspired Frances Scott Key’s “Defense of Ft. McHenry,” later known as “The Star-Spangled Banner.”

American forces commanded by General Andrew Jackson prevented invading British forces from seizing the port of New Orleans.

American ratification of the Treaty of Ghent, successfully negotiated the previous December and ratified by Britain on December 27, 1814, ended the War of 1812.

Matthew Vassar was elected a trustee of the village of Poughkeepsie.