In 1853, he became Deputy Postmaster of Watertown, New York, at a salary of $600 a year, and after six years had saved $1,000 and opened with a partner a jewelry store. Two years later, he bought his partner out, and continued in this business until 1869. In 1869, Henry Keep, a former President of the New York Central Railroad, was dying and asked Roswell Flower, whose wife was a sister of Keep"s wife Emma, to manage the $4,000,000 estate for his widow.
He was elected as a Democrat to the 47th United States Congress to fill the vacancy caused by the resignation of Levi P. Morton upon his appointment as Minister to France, and served from November 8, 1881, to March 3, 1883.
He was also elected to the 51st and 52nd United States Congresses, and served from March 4, 1889, to September 16, 1891, when he resigned upon his nomination for Governor. He was Governor of New York from 1892 to 1894, elected in 1891, the last one to serve a three-year term.
During his term he signed into law the creation of the City of Niagara Falls. He died of a heart attack on May 12, 1899 in Eastport, New York at the Long Island Country Clubhouse.
A monument to Flower, designed by noted sculptor Augustus Saint-Gaudens in 1902, is located on lower Washington Street at Watertown.
lieutenant is in the Public Square Historic District, listed on the National Register of Historic Places in 1984.
Keep replied, "He is as honest a man as there is in the State of New York, but for fear that somebody else will cheat, he will always begin first." The business brought Flower to New York City where he became known as a shrewd financial administrator, and opened the banking house of R. P. Flower & Company