Afterward Spencer attended Yale College, where he was an editor of the student newspaper and participated in Lacrosse. He graduated with honors, seventh in a class of one hundred fifty, in 1884.
A Republican, he served as a United States Senator from Missouri. His great-grandfather was American Revolutionary War veteran Lieutenant Israel Selden Spencer, who participated in the Battle of Saratoga and others Selden P. Spencer received his basic education in Erie before attending Hopkins School, a college preparatory school in New Haven, Connecticut.
He then moved to Saint Louis, Missouri to attend Washington University law school, graduating in 1886.
Admitted to the bar in 1886, Spencer opened a law practice in Saint Louis with future Missouri governor Forrest Donnell while also serving as a professor of medical jurisprudence at the Missouri Medical College. The college would later bestow him with an honorary Doctor of Medicine degree in appreciation of his efforts.
Westminster College in Fulton, Missouri also granted him honorary Doctor of Philosophy and LL.D degrees. While in the Missouri House he served as Chairman of the Committee on Banks and Banking, as well as on the Judiciary, Ways and Means, Militia, and Rules Committees.
From 1897 to 1903 he was a judge of the United States circuit court.
At the end of his term on the court Spencer returned to his law practice. He also became heavily involved with the American Bar Association, serving on its executive board and as vice-president in 1914. During World War I he served as chairman of a Saint Louis area draft board.
The unexpected death of Missouri United States. Senator William J. Stone in April, 1918 prompted Selden Spencer"s return to political office.
Xenophon P. Wilfley was appointed a temporary replacement until a special election could be held. In November, 1918 Spencer defeated former Governor Joseph West. Folk with 52-percent of the vote to fill the remaining two years of Stone"s term.
Senator Spencer was also noted for being one of the Republicans in opposition to the Treaty of Versailles and America"s participation in the League of Nations, working with Senator Henry Cabot Lodge and the Irreconcilables. Senator Spencer made numerous speeches against the treaty while campaigning for fellow Republicans in 1920 and 1922.
Senator Selden P. Spencer died at Walter Reed Hospital in Washington, District of Columbia on May 16, 1925 following complications from hernia surgery.
He is buried in Bellefontaine Cemetery in Saint Louis.
Selden Spencer first held elected office in 1895 when he was voted a member of the Missouri House of Representatives. While in the Senate, he was chairman of the Committee on Claims (Sixty-sixth and Sixty-seventh Congresses) and a member of the Committee on Indian Affairs (Sixty-seventh Congress) and the Committee on Privileges and Elections (Sixty-seventh through Sixty-ninth Congresses).