Isaac Forsyth was the eldest son of Alexander Forsyth and Rachel Lindsay. He was referred to as Major in a letter by a descendant, but this seems unlikely. Tradition also says he served George Washington and Lafayette. I have found sources mentioning an Isaac Forsyth from Philadelphia who was about a generation older who advanced in the military to Sergeant. It is possible that someone confused the two individuals. Our Isaac is only found as a Cornet in the Hanover Troop of Horse for a short period.
    According to the The 1810 census record for Bourbon County, Kentucky, Isaac was born before 1766. The 1800 census record for Heidelberg Township, York County, Pennsylvania lists Isaac as being born before 1775. It is known from her obituary, that his mother, Rachel Lindsay, was born about 1750/51. Rachel's father, David Lindsay, first appears in the records in Charles Town, Chester County, Pennsylvania in 1768. It is not until the following year that Isaac's father, Alexander Forsyth, is listed. Alexander Forsyth is listed as an inmate, indicating that he was married, but did not own land. Alexander and Rachel must have married perhaps around 1765 when she was very young leaving room for Isaac to have been born in late 1765 or early 1766.
    Isaac died between March 6, 1830 when he received payment from his father's estate, and October 3, 1831 when his father's will was probated (Isaac was not listed). There is no record of his death in Bourbon County, and he apparently died intestate (without a will). He married on December 22, 1792, at the First Presbyterian Church in Baltimore, Maryland to Francis Brown. They were married by Rev. Allison. Francis Brown died before June 19, 1803, when Isaac married a second time to Nancy Ann Letton in Prince George's Parish in Montgomery County, Maryland. Isaac bought property from Richard McAllister's estate in Hanover in 1804, so presumably Francis died in Hanover. There is no evidence that Isaac every lived in Baltimore.

Isaac and Francis had 4 children:

    Bethilda "Matilda" Forsyth (10 Apr 1792 - aft 1820) m. Cornelius Rayburn
    Augustus "Gustie" Forsyth (7 May 1794 - ) m. Frances Sparks
    Oliver Forsyth (abt 1796 - bet 1830 and 1 Apr 1838) m. Margaret Layson
    Denmark Forsyth (5 Sep 1800 - 16 Sep 1876) m. Abigail Brown

    Bethilda was born 9 months prior to her mother's marriage to Isaac. Some descendants have referred to Isaac's first wife as Francis Nut Brown. In the 1790 census of Baltimore, Isaac's father Alexander is found living 13 dwellings from head of household, Nut Brown, with a wife and 2 daughters.
    In the winter of 1805/06, Isaac and his second wife Ann moved to Paris, Bourbon County, Kentucky. He owned a farm in what is now Harrison County, Kentucky. Isaac was a farmer, trader, and woodcraftsman. They eventually settled in Jackstown on the Licking River with twelve children. Isaac is listed in the 1820 Kentucky Census in Bourbon County.
    Ann Letton, the daughter of Michael Letton, was born March 21, 1786, in Rockville, Maryland, and died May 3, 1876, in Jackstown, Kentucky. She is mentioned in Michael Letton's will (Montogomery Co. Will Book L pg 117). Micheal Letton left a slave "under guardianship of Isaac Forsythe" to Avelina in his will, probated in 1819. Michael Letton was also the bondsman of Bethilda when she married in 1816. Some source say that Ann Letton had been previously married and that the eldest two children are from that marriage, however, the oldest, Avelina, was born over a year after she married Isaac Forsyth, when Ann was just 22 years old, so that claim seems unlikely.

Isaac and Anne had 8 children were:

    Avoline Letton Forsyth (28 Aug 1804 - ) m. _____ Millirre
    Wayne Thompson Forsythe (1808 - 1880) m. Evalina Hutchinson
    Anne Gay "Angelina" Forsyth (20 Mar 1811 - 12 Dec 1876) m. 1828/32, Daniel Ashby Million
    Elvira W. Forsyth (5 Jun 1816 - 24 Dec 1888) m. 22 Nov 1837, Benjamin L. Hayth
    Viviani Ann Forsyth (19 June 1819 - abt 2 Apr 1888) m. Hamilton J. Hulcee
    January Hartt Forsythe (1821 - Dec 1874) m1. Patsey M. Baker, m2. 1863, Margaret A. Redmon
    Omelia Forsyth (abt 1823 - ) m1. 1 Sep 1835, Israel Lindley, m2. 22 Oct 1851, John Thomas Steffee
    Alexander Forsyth ( - after 29 June 1829)

A posting on the Internet from Helen Barricklow Endecott (1927-2005) on Ann Letton's daughter Avelina says:

"Avelina Le Forsythe's will dated 29 June 1829. She names her mother (name unreadable), her four sisters: Angalar, Amelia, Bioiami? & Elvird. Her Uncle Caleb Letton. Brothers, January & Alexander Forsythe. She obviously died young and unmarried. Her father is probably deceased since she had an estate which was probably her share from her father. Bourbon Co., Ky."

Since Isaac is not mentioned in her will, it may indicate that he had died prior to June 29, 1829.