Some significant structures are among those located within the Historic District including the DeKalb County Courthouse and the Sycamore Public Library.

Some of the major structures include several prominent Queen Anne style mansions, the Sycamore library, the DeKalb County Courthouse, and dozens of mid- to late 19th-century houses.

The Department of Conservation dispatched field surveyors to all Illinois counties to find anything that might qualify for inclusion on the National Register of Historic Places.

[3] The field surveyor who traveled to Sycamore in 1973 found a large number of late 19th and 20th century examples of architecture that he recommended the establishment of a historic district.

The mayor of Sycamore then appointed a citizens' committee to assist the state with the work involved in listing the district.

[3] The boundaries were drawn up by Robert Wagner, a Chicagoan who worked as a National Register assistant with the Illinois Department of Conservation.

Wagner drew up the boundaries on the basis of "visual integrity"..[3] This led to an irregular pattern of inclusion for the Sycamore Historic District.

In other cases, such as with the library or the U.S. Post Office an arbitrary line was drawn with the sole purpose of including a specific structure.

The largest concentration of commercial contributing properties to the historic district are found along Illinois Route 64 as it passes through Sycamore.

The Courthouse sits in the center of a square facing Illinois Route 64, directly across the north/south street, Main, from the Sycamore Public Library.

[8] Though quite altered from its original state in the mid 19th century George's Block remains one of the more eye catching structures in the Historic District.

[9] In the first year the building existed such famous men as Horace Greeley, Charles Sumner and Bayard Taylor spoke there.

[9] The houses in the Sycamore Historic District cross a variety of architectural styles and span from the 1830s to the early 20th century.

The library, still operational today, was constructed in 1905 with a combination of philanthropical gifts from different sources including Andrew Carnegie.

These factors, along with the growth of mature shade trees, combine to give the streetscape a well-balanced and integrated look and feel.

The buildings themselves are most significant for their architecture which contributes to what the National Register of Historic Places nomination form in 1978 said conveyed "a gracious calm very close to the popular American image of an ideal small town.