In contrast with the airy, iron-framed interior, the library's exterior is very reminiscent of Italian Cinqueccento Palazzi. The facade design is made up of narrow arched windows which punctuate the stolid, unadorned stone walls of the base. A sculptured garland enlivens the transition to the upper floor, which is rhythmically composed of a series of inscribed arches framing lunette windows. Panels within the arches bear the names of hundreds of authors whose works were to be contained inside. These panels were socially modern, since Labrouste chose to celebrate not only the individuals in society but these individuals coming together to form a union. This union taking the form of a public place of knowledge and learning. The concept of a union during a time of the revolution of 1848, turned many heads, but Labrouste was used to controversy ever since his days as a student in Rome.

The exterior of the library reinforced the independent nature of the building. Since it was being built during a time of revolution, Labrouste designed the building to be able to withstand attack or arson, if it were ever targeted by the revolutionaries. During this time of civil turmoil, the library's design was heavily influenced towards a building which could stand alone and be protected from all sides. The simple, indeed brutal, style was seen as appropriate to a time when buildings needed only to be purely functional, with the aesthetic beauty only present on the safe interior of the building. This long, rectangular and precise building lead to the railway architecture style which began to spring up later in Paris. Defensive planning for the library was the main effect of the revolution of 1848 on Labrouste's design of the library. As so much civil and political turmoil was unfolding around Paris, and around Europe, Labrouste designed a building that not only could withstand the chaos, but stood as a symbol of unity and sanity.

The reason the exterior of the building was considered so modern, was its simplicity of design. Modern architecture relies on function over form. As Labrouste said himself, " The beauty of a monument resides in the expression of a harmony between needs and means used to satisfy them". The library then is a perfect harmony between its needs and the means to satisfy them. One example of this is, Labrouste's decision to supply gas power to light the library at nights, so the public could use the resources all through the night. Labrouste used the combination of cast-iron, steel, concrete and stone to make the building completely fire proof. Also the use of large glass panels for most of the ceiling was a perfect example of the marriage of form and function, as the glass was a lighter material to support as a ceiling, and provided more natural light for the second floor reading room in the library.

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