Cady designed the American Museum of Natural History, the Metropolitan Opera House, and fifteen buildings at Yale University.

[12] He advertised that he could provide designs and plans for churches, cottages, public buildings, residences, schools, stores, and warehouses.

[12] The Brooklyn Union described Cady as "a young man of fine talents, of refined and cultivated taste, and profoundly zealous in his profession".

[13] From 1864 to 1881, his offices were in the Trinity Building at 111 Broadway in Manhattan which served as a studio for dozens of architects, providing opportunities for many collaborations.

[14] In 1880, Cady designed an elegant Old English double house at 1826 Massachusetts Avenue in the Northwest neighborhood of the District of Columbia for sisters Mrs. Katherine Miller and Mrs. Charlotte E. Hopkins.

[19] In 1920, Alexander Dana Noyes wrote, "In his professional career, J. Cleveland Cady was perhaps the embodiment of the effort of American architecture, fifty years ago, to find itself while cutting loose from the false and meretricious standards of the Second Empire.

[24] Cady designed the original Peabody Museum of Natural History to house Yale University's mineral collection, fossils, and exhibits on zoology and geology.

[28] This was likely the first time Cady used Richardson Romanesque style and is a restrained attempt because he wanted to blend the library into its rural background.

[30][4]: 18  Its four-story façade had matching seven-story towers with commercial establishments on the ground level, restaurants and ballrooms above, and apartments for bachelors in the upper levels—Korom calls this "an early and excellent example of a multi-use project".

[32] Although its yellow brick and terracotta exterior was not particularly noteworthy, the Historic American Buildings Survey noted, "its interior placed it among the great opera houses of the world".

[30][4]: 19–20  However, in 1939, the New York City Guide by the Federal Writers' Project, noted, "The opera house was designed by J. C. Cady, a prominent architect of the day.

That Mr. Cady was without experience in theater construction seemed to matter little; audiences ever since have paid for his mistakes, as but half the stage can be seen from the side seats of the balcony and family circle.

[4]: 23  Running 710 feet (220 m) along the museum's West 77th Street entrance, this was the longest public building façade in New York City.

[4]: 23–24  It included black cherry window frames, elaborate cornices, cartouches, eagles, finials, wreaths, and a 112 feet (34 m) wideporte-cochère.

[36] Joan Kelly Benard says, "the 77th Street façade has been hailed as one of the finest examples of Romanesque Revival architecture in New York City.

"[36] Inside, the building reflected a new approach to museum design; it included spaces for public exhibitions and scholarly study.

[37] Robert Habersham Coleman, a former classmate and fraternity brother of Cady, donated the funds to construct the chapter house.

[39][40][4]: 31 Cady's brick and masonry Romanesque Revival design for North Sheffield became the template for Winchester and Watson Halls.

[4]: 11 George Brush, head of Sheffield's engineering department, praised North Sheffield Hall, saying:The building is considered a complete success; great surprise is expressed that with so simple an external form—a mere cube—such an admirable architectural effect has been produced, and the interior arrangements are so simple, complete and substantial, that everyone is impressed with the fact, that nothing has been sacrificed to mere decoration, but everything is for use...it is thoroughly well adapted for the uses of our institution.

[4]: 12 Cady's design for the Yale Infirmary (1892) departed from his simple institutional buildings and instead looked like a Neocolonial or Georgian revival style mansion to create a refuge for sick students.

[4]: 31, 33  He designed the Renaissance Revival style Hendrie Hall (1894–1900) for Yale's Law School; it now houses the Adams Center for Musical Arts.

[47] He also designed the brick and wood Berea College Hospital (1908, razed 1954) which was turned into a dormitory called Morningside Cottage in 1925.

[4]: 27  A Presbyterian, Cady avoided the traditional Anglican and Catholic cruciform plan but instead used the English "dissenting chapel" arrangement.

[54] Inside, the wood trim is adorned with carved African American faces, and the yellow pine pews were built by students from the Hampton trade school.

[60] The New York Association for Improving the Condition of the Poor (AICP) hired Cady, Berg & See to design the People's Bath at 9 Centre Market Place in the Lower East Side.

[61] The AICP criticized New York City and the architects for what they called "a pretentious brick structure", finding it too grand, too large, and too expensive for the bathhouse scheme they had in mind.

[29][68] The New Haven Preservation Trust says the "fortress-like mansion" merges Jacobean revival style with some Queen Anne details.

In 1876, Cady designed Cliffside, a Dutch Colonial Revival style stone mansion in Palisades, New York, for Lynda and Henry Lawrence.

[72] Cady was Richard Morris Hunt's biggest rival for the Tribune Tower competition; although the latter eventually won and designed the building in New York.

[74][75] In a letter to her parents, his niece Lavinia Goodell wrote, "Poor Cleveland will feel the loss very deeply for he thought everything of her.