Costigan designed the mansion in the Greek Revival style, popular in the mid 19th-century, drawing from the pattern books of New York architect Minard Lafever.

[3] Construction of the house used materials that were manufactured nearby, or on the premises, including the brick, limestone and timber, largely tulip poplar, that was plentiful in the area.

The mansion would remain within the Lanier family until 1917, when James' youngest son Charles donated the site to the Jefferson County Historical Society.

The southern river front features a two-story portico that extends the length of the facade, enclosed with ornate iron railings, and supported by four 30-foot fluted wooden columns with carved Corinthian capitals.

A plain wooden parapet showcasing a decorative scroll ornament encompasses the low-hipped gabled roof that is crowned by an octagonal shaped cupola.

The center bay features a recessed paneled doorway with full length side lights, and framed with a classical surround of pilasters supporting a molded entablature.

Due to the sloping nature of the site, the raised foundation is not as pronounced on the northern, city front elevation as opposed to the southern facade.

The city front elevation mimics the southern front in regards to window, door placement and trim, but instead of an ornate two-story portico there is only a central one-story portico supported by a pair of fluted Corinthian columns, and plain squared pilasters that surrounds the entrance, flanked by wrought iron balconies under the respective lower windows.

The adjacent parlors are largely furnished as one, with twin black marble fireplaces, gilded mantel mirrors and chandeliers that hang from the 14 feet (4.3 m) high ceilings.

[7] The library with its highly stylized decorative ceiling done in gold leaf, deep red and blue, is separated from the dining room by a passageway that leads to the service wing.

The dining room features a botanical wallpaper from the French manufacturer Zuber, that uses a technique called British Transfer, where each color in the pattern is separately stamped onto the paper.

The upper stories are accessed by a self-supporting staircase, trimmed with a Greek key design, found at the midpoint of the entrance hall that corkscrews up to the cupola.

The third floor, plainly decorated and with ceilings just over 6 feet (1.8 m), housed smaller children, additional servants quarters and storage space.The Lanier Mansion sits on a large rectangular lot encompassing 10 acres (0.040 km2) within the city of Madison.