Arlington was once an independent community governed unofficially by the Arlington Community Club and Duval Co. It was annexed in 1968* as a part of Jacksonville, Florida. What is now known as Arlington is made up of several early neighborhoods. You will see references to neighborhoods (such as Eggleston) as if they are somewhere outside of Arlington. It is due to the commonly understood boundaries of Arlington changing over time (meaning that the area known as Arlington expanded). These interpretations effectively extended what was known as Arlington.
Today, what is known as the Greater Arlington area (but referred to as Arlington) extends from the west boundaries bordering on the St. John's River to the farthest eastern boundaries of the peninsula near Mayport. In the earliest days, Arlington was considered to be only the westernmost portion, and ending at Mill Creek. Within even that boundary, neighborhoods such as Clifton, Floral Bluff, Eggleston, Gilmore, and Chaseville have had their own identity started when those neighborhoods were first named. The definition of what we consider "Old Arlington" remains to be the earliest definitions of Arlington.
The original "Arlington" area was bordered by the St. Johns River on the west and northern edges, and slightly south of the Arlington River on the southern edge. Access to the waterways was important so the earliest establishments developed along these edges. The westernmost edge was directly across from what is now downtown Jacksonville (originally known as "Cows Ford" due to it being an area of the St. John's River where cattle could be driven across).
As a result of the growing development of the area in Arlington, a ferry was built connecting Arlington's westernmost edge to downtown Jacksonville. The area around the Arlington end of the ferry naturally developed faster than other parts of the peninsula that Arlington is on. This older and more popular area has become the focus of the area we call "Old Arlington." Mill Creek Road is considered the eastern edge of "Old Arlington." East of Old Arlington is naturally known as "Arlington East."