(September 3, 1856–April 14, 1924) was an American architect, called the "father of modernism". He is considered by many as the creator of the modern skyscraper, was an influential architect and critic of the Chicago School, and was a mentor to Frank Lloyd Wright.

Louis Sullivan was born in Boston, to an Irish-born father and a Swiss-born mother both of whom immigrated to the United States in the late 1850s. While attending high school Sullivan met Moses Woolson, whose teachings made a lasting impression on him, and nurtured him until his death. After graduating from high school, Sullivan studied architecture briefly at the Massachusetts Institute of Technology. Learning that he could both graduate from high school a year early and pass up the first two years at the Massachusetts Institute of Technology by passing a series of examinations, Sullivan entered MIT at the age of 16. After one year of study, he moved to Philadelphia and talked himself into a job with architect Frank Furness.

The Depression of 1873 dried up much of Furness’s work, and he was forced to let Sullivan go. At that point Sullivan moved on to Chicago in 1873 to take part in the building boom following the Great Chicago Fire of 1871. He worked for William LeBaron Jenney, the architect often credited with erecting the first steel-frame building. After less than a year with Jenney, Sullivan moved to Paris and studied at the École des Beaux-Arts for a year. Renaissance art inspired Sullivan’s mind, where he was influenced to relate his architecture to emulate Michelangelo's spirit of creation rather than replicate the styles of earlier periods. He returned to Chicago, not yet 18 years old. He began work for the firm of Joseph S. Johnston & John Edelman as a draftsman. Johnston & Edleman were commissioned for interior design of the Moody Tabernacle, which was completed by Sullivan.[1] In 1879 Dankmar Adler hired Sullivan; a year later, he became a partner in the firm. This marked the beginning of Sullivan's most productive years.

With a string of triumphs such as the 1889 Auditorium Building in Chicago (where Adler and Sullivan reserved the top floor of the tower for their own atelier), the 1891 Wainwright Building in St. Louis and the 1899 Carson Pirie Scott Department Store on State Street in Chicago, Louis Sullivan was the first architect to find the right form for a steel high-rise. Louis Sullivan lived by the phrase "form follows function". The steel girder was the form for the steel high-rise. This new way of constructing buildings pushed them up rather than out. The steel frame allowed taller buildings with larger windows, which meant more interior daylighting, and more usable floor space. The technical limits of masonry had always imposed formal constraints; those constraints were suddenly gone. None of the historical precedents were any help, and this new freedom created a kind of technical and stylistic crisis.

Sullivan was the first to cope with that crisis. He addressed it by embracing the changes that came with the steel frame, creating a grammar of form for the high rise (base, shaft, and pediment), simplifying the appearance of the building by using ornament selectively, breaking away from historical styles and using his own intricate flora designs, using that ornament in vertical bands to draw the eye upwards and emphasize the building's verticality, and relating the shape of the building to its purpose. All this was revolutionary, appealingly honest, and commercially successful.

In 1890 Sullivan was one of the ten architects, five from the Eastern US and five from the Western US, chosen to build a major structure for the "White City", the World's Columbian Exposition, held in Chicago in 1893. Sullivan's massive Transportation Building and huge arched "Golden Door" stood out as the only forward-looking design in a sea of Beaux-Arts historical copies, and the only gorgeously multicolored facade in the White City. Sullivan and fair director Daniel Burnham were vocal about their displeasure with each other. Sullivan was later (1922) to claim that the fair set the course of American architecture back "for half a century from its date, if not longer." (Autobiography of an Idea, p. 325) His was the only building to receive extensive recognition outside America, receiving three medals from the Union Centrale des Arts Decoratifs the following year.

Adler and Sullivan broke their partnership after the Guaranty Building. Afterwards Sullivan went into a twenty-year-long financial and emotional decline, beset by alcoholism and chronic financial problems. He was awarded several commissions for small-town Midwestern banks (see below), wrote books, and in 1922 appeared as a critic of Raymond Hood's winning entry for the Tribune Tower competition, a steel-frame tower dressed in Gothic stonework that Sullivan found a shameful piece of historicism. He and former understudy Frank Lloyd Wright reconciled in time for Wright to help fund Sullivan's funeral after he died, poor and alone, in a Chicago hotel room. A modest headstone marks his final resting spot in Graceland Cemetery in Chicago's Uptown neighborhood.

Sullivan's legacy is contradictory. He is the first modernist. His stripped-down, technology-driven, forward-looking designs clearly anticipate the issues and solutions of Modernism. In his last years, Sullivan seemed willing to abandon ornament altogether in favor of honest massing. But to experience Sullivan's built work is to experience the irresistible appeal of his incredible designs, the vertical bands on the Wainwright Building, the burst of welcoming Art Nouveau ironwork on the corner entrance of the Carson Pirie Scott store, the (lost) terra cotta griffins and porthole windows on the Union Trust building, the white angels of the Bayard Building. Except for some designs by his long time draftsman George Grant Elmslie, and the occasional tribute to Sullivan such as Schmidt, Garden & Martin's First National Bank in Pueblo, Colorado (built across the street from Adler and Sullivan's Pueblo Opera House), his style is unique. A visit to the preserved Chicago Stock Exchange trading floor, now at The Art Institute of Chicago, is proof of the immediate and visceral power of the ornament that he used so selectively. Original drawings and other archival materials from Sullivan are held by The Art Institute of Chicago and by the Drawings and Archives Department in the Avery Architectural and Fine Arts Library at Columbia University. Fragments of Sullivan buildings are also held in many fine art and design museums around the world.