It is correct that Tennessee became a state in 1796, but Sumner County's records predate that by
about 10 years. Sumner County was originally created in 1786 as a North Carolina County, and
has never ceased to function as a county since that time. In 1790, North Carolina ceded its
western territory to the United States, and what later became Tennessee was set up as the
Territory of the United States South of the Ohio River (or Southwest Territory for short). When
the Territory was admitted to the Union as the State of Tennessee, this still didn't change
anything regarding Sumner County's existence.
Prior to 1786, what is now Sumner County was in Davidson County, so some very early records for people and property in Sumner can be found in Davidson. Davidson County was created in 1783, and officially prior to that time Middle Tennessee would have been in Washington County, North Carolina (now Washington County, Tennessee), so you might check records there. However, it was so far to Jonesboro, the seat of Washington County, in the 18th Century, I doubt if a great deal is included in their records for the Cumberland settlements.
Remember too that in 1799, Wilson and Smith Counties were cut off from Sumner County on the South and East. Macon County came off in the 1830s to the East, and Trousdale County was created in 1870, also on the Southeast corner.
One other tip: when looking for the original land grant holder in the northern part of Sumner County, north of what was called Walker's line, you have to check Kentucky land grant records. (There is an old book by Jillson that lists them all).Lots of land in northern Sumner County was actually granted by the State of Kentucky, not Tennessee. The reason for this was a long standing boundary dispute between the two states, and this was a compromise. That applies to most of the counties that are along the Kentucky boarder, not just Sumner.