In 1850 he traveled to Europe to visit the many public gardens found there. After returning from Europe he wrote Walks and Talks of an American Farmer in England which also helped launch his career as the pioneer of landscape architecture in the United States.
Olmsted’s friend and mentor, Andrew Jackson Downing, the landscape architect from Newburgh, New York first proposed the development of Central Park as publisher of The Horticulturist magazine. It was Downing who introduced Olmsted to English-born architect Calvert Vaux; Downing had died a tragic death in 1852 and in his honor Olmsted and Vaux entered the Central Park design competition together and won. They continued in informal partnership to design Prospect Park in Brooklyn from 1866 to 1868 and then onwards to other projects. Vaux remained in the shadow of Olmsted's grand public personality and social connections.
After completing the Central Park project Olmsted served as Executive Secretary of the U.S. Sanitary Commission, a precursor to the Red Cross in Washington D.C which tended to the wounded during the Civil War. After the war he managed the Mariposa mining estate in the Sierra Nevada mountains in California. In 1865 Vaux and Olmsted formed Olmsted, Vaux and Company. When Olmsted returned to New York, he and Vaux designed Prospect Park, Chicago's Riverside subdivision, Buffalo, New York's park system, and the Niagara Reservation at Niagara Falls.
In 1883 Olmsted established his landscape architecture firm in Brookline, Massachusetts, where he designed Boston's Emerald Necklace and the 1893 World's Fair in Chicago among many other of his projects. In 1895, senility forced him to retire. He moved to Waverly, Massachusetts and took up residence at McLean Hospital which he had landscaped several years before.