Isaac appears as a witness on probate records in Cayuga County, New York, and administered his father's estate there.
The History of Cayuga County states: Sennett Baptist Church was organized September 12th, 1799, as the Third Baptist Church in Aurelius, and Thomas Morley represented them in the Scipio General Conference, of which they became a member, September 25th, 1799. In 1801 they experienced a revival, by which fifty-one were added to their number, making their total membership at that time eighty-six. In this year the Cayuga Baptist Association was organized in the barn of Ebenezer Healy. The delegates attending that meeting were Manasseh French, Messrs. Squire, Manro, Thomas Morley, Smitten Irish, John Jeffries, Israel Clapp and Isaac Barnum. Up to 1803, the average annual increase was thirty-one.
According to a list of the first officers of Jefferson, now Mentz, who were elected March 6th, 1804, Caleb Ward and Isaac Barnum were Overseers of the Poor.
An undated article from the Courier-Leader, Paw Paw, Michigan states: "In times past - Isaac Barnum of York State is known to have settled in Clinch Township in 1835. In visiting with Hortense Cornell of Paw Paw, a direct descendant of Isaac, it was learned that Isaac's son, Edmond [N.B. s/b Edwin], came here first, walking from York State. He first settled west of Paw Paw. Others of the family may have settled there later as it is from this family that Barnum School got its name. Isaac Barnum planned to start a mill in this area (with Myers by other accounts), and needed water power. Edmond, in scouting around, found a likely creek in Clinch Township in the SE 1/4 of Sec. 29 of what later became Almena Township. Edmond wrote home and soon Mr. and Mrs. Barnum and family, consisting of eight or more children and some in-laws, were on the way. They shipped their household goods by sailing vessel throught the Lakes to South Haven. The family came over-land by stagecoach. When the ship arrived at Sough Haven the waters were so rough that capain thought it best to throw the cargo overboard to insure the safety of his passengers. So the Barnums retrieved only a few broken pieces of furniture, which were washed ashore. Miss Cornell has three drawers built into the diningroom of her home which were thus retrieved.
"Two months at their new home Isaac died, leaving his widow and sons to carry on with his dream. One Barnum daughter (Hannah) married Asahel Downing who came with the family from York State. Several in Almena are descended from this family. Another daughter (Ophelia) married Calvin Hawley, the ancestor of Miss Cornell."
His obituary, from an unidentified contemporary newspaper, states: Herkimer, N.Y. - Died [1937 written in pencil] - Paw Paw, Van Buren County (Michigan) Jun. 4, of numb palsey, Isaac Barnum, Esq. aged 62 years. The deceased removed from the town of Mentz, Tomkins county, last September, where he had resided rising of thirty years. He has left a large circle of friends and relations to lament his sudden and unexpected death. The Barnum Family, 1517-1904 says that he died in 1836.