Morrison was elected as a Democrat to the Sixty-first and to the three succeeding Congresses (March 4, 1909 – March 3, 1917).
He served as chairman of the Committee on Patents (Sixty-fourth Congress).
He was not a candidate for renomination in 1916, but instead resumed the practice of law.
He served as president of the United States Civil Service Commission from March 1919 to July 1921 and became a member of the legal staff of the chief counsel of the Federal Trade Commission at Washington, D.C., on December 10, 1925, and served until his retirement on April 30, 1942, maintaining his residence in Washington, D.C.