Always active and interested in politics (FDR was a great admirer of his distant cousin, Theodore Roosevelt, who served as President of the United States from 1901 to 1908), FDR abandoned law in 1910 to run for the New York State Senate. FDR ran as a progressive, independent-minded Democrat, who stood in staunch opposition to the "political bossism" so prevalent at the time. He won election by a comfortable margin, and would serve in the New York State Senate for the next three years, having won re-election in 1912. As a state senator, FDR continued his opposition to "machine" politics," sponsored a resolution urging New York's congressional delegation to approve the Seventeenth Amendment to the U.S. Constitution calling for the direct election of senators, and was an early champion of the cause of conservation.

FDR's career in the New York State Senate came to an end in 1913, when, as a reward for his support of the Woodrow Wilson's presidential candidacy at the Democratic National Convention in 1912, he was appointed Assistant Secretary of the Navy. FDR was thrilled. The Navy Department was one of the largest and most important government agencies and it offered FDR substantial opportunities to gain valuable administrative experience and make important political contacts from coast to coast. The appointment also seemed propitious. His cousin Theodore held the post in 1898 and had used it as an effective stepping stone in his march to the governorship of New York, and finally the White House. FDR threw himself into the position with great enthusiasm, and soon established a reputation as an energetic and effective administrator. Granted considerable latitude by his superior, Josephus Daniels, FDR focused his attention for the most part on the business side of the Navy Department, although he did, on occasion, discuss tactics. After the U.S. entry into World War One, for example, FDR pressed Secretary Daniels to rush through a crash building program of 50-foot launches to defend U.S. ports against the German submarine menace -- a program that Daniels rejected. FDR was more successful in his promotion of the so-called North Sea Mine Barrage, an ambitious plan designed to keep German submarines out of the North Sea by sewing a "belt" of mines from Norway to Scotland. Like the proposed 50-foot launch scheme, Daniels also opposed this idea, but after a direct appeal by FDR to President Wilson, the plan was approved. In the Spring of 1918, the British and American Navies began the difficult task of laying the mines, and although the barrage remained incomplete at War's end, it limited German access to the North Sea and was a factor in the collapse of morale among German sailors that manifested itself in the famous Kiel mutiny of November 1918.