After graduation, Mahony returned to Chicago, where she became the first woman to be licensed to practice architecture in Illinois.
She worked in her cousin's architecture firm, which was located in Steinway Hall at 64 E. Van Buren in downtown Chicago.
The space was shared with many other architects, including Robert C. Spencer, Myron Hunt, Webster Tomlinson, Irving Pond and Allen Bartlitt Pond, Adamo Boari, Birch Long and Frank Lloyd Wright.
In 1895, Mahony, the first employee hired by Frank Lloyd Wright, went to work designing buildings, furniture, stained glass windows and decorative panels.
Her beautiful watercolor renderings of buildings and landscapes became known as a staple of Wright's style, though she was never given credit by the famous architect.
Over a century later she would be known as one of the greatest delineators of the architecture field, but during her life her talent was seen as only an extension of the work done by male architects.
She was associated with Wright's studio for almost fifteen years and was an important contributor to his reputation, particularly for the influential Wasmuth Portfolio, for which Mahony created more than half of the numerous renderings.
Architectural writer Reyner Banham called her the "greatest architectural delineator of her generation."
Her rendering of the K. C. DeRhodes House in South Bend, Indiana, was praised by Wright upon its completion and by many critics.

Wright understated the contributions of others of the Prairie School, Mahony included.
A clear understanding of Marion Mahony's contribution to the architecture of the Oak Park Studio comes from Wright's son, John Lloyd Wright, who says that William Drummond, Francis Barry Byrne, Walter Burley Griffin, Albert Chase McArthur, Marion Mahony, Isabel Roberts and George Willis were the draftsmen—the five men and two women who each made valuable contributions to Prairie style architecture for which Wright became famous.
During this time Mahony designed the Gerald Mahony Residence (1907) in Elkhart, Indiana for her brother and sister-in-law.

When Wright eloped to Europe with Mamah Borthwick Cheney in 1909, he offered the Studio's work to Mahony but she declined.
After Wright had gone, Hermann V. von Holst, who had taken on Wright's commissions, hired Mahony with the stipulation that she would have control of design.
In this capacity, Mahony was the architect for a number of commissions Wright had abandoned.
Two examples were the first (unbuilt) design for Henry Ford's Dearborn mansion, Fair Lane and the Amberg House in Grand Rapids, Michigan.
After graduation, Mahony returned to Chicago, where she became the first woman to be licensed to practice architecture in Illinois. She worked in her cousin's architecture firm, which was located in Steinway Hall at 64 E. Van Buren in downtown Chicago. The space was shared with many other architects, including Robert C. Spencer, Myron Hunt, Webster Tomlinson, Irving Pond and Allen Bartlitt Pond, Adamo Boari, Birch Long and Frank Lloyd Wright. In 1895, Mahony, the first employee hired by Frank Lloyd Wright, went to work designing buildings, furniture, stained glass windows and decorative panels. Her beautiful watercolor renderings of buildings and landscapes became known as a staple of Wright's style, though she was never given credit by the famous architect. Over a century later she would be known as one of the greatest delineators of the architecture field, but during her life her talent was seen as only an extension of the work done by male architects. She was associated with Wright's studio for almost fifteen years and was an important contributor to his reputation, particularly for the influential Wasmuth Portfolio, for which Mahony created more than half of the numerous renderings. Architectural writer Reyner Banham called her the "greatest architectural delineator of her generation." Her rendering of the K. C. DeRhodes House in South Bend, Indiana, was praised by Wright upon its completion and by many critics.
Wright understated the contributions of others of the Prairie School, Mahony included. A clear understanding of Marion Mahony's contribution to the architecture of the Oak Park Studio comes from Wright's son, John Lloyd Wright, who says that William Drummond, Francis Barry Byrne, Walter Burley Griffin, Albert Chase McArthur, Marion Mahony, Isabel Roberts and George Willis were the draftsmen—the five men and two women who each made valuable contributions to Prairie style architecture for which Wright became famous. During this time Mahony designed the Gerald Mahony Residence (1907) in Elkhart, Indiana for her brother and sister-in-law.
When Wright eloped to Europe with Mamah Borthwick Cheney in 1909, he offered the Studio's work to Mahony but she declined. After Wright had gone, Hermann V. von Holst, who had taken on Wright's commissions, hired Mahony with the stipulation that she would have control of design. In this capacity, Mahony was the architect for a number of commissions Wright had abandoned. Two examples were the first (unbuilt) design for Henry Ford's Dearborn mansion, Fair Lane and the Amberg House in Grand Rapids, Michigan.
Marion Mahony Griffin