Loomis was a member of the Connecticut House of Representatives in 1851 and a delegate to the Republican National Convention in 1856. In addition, he was a member of the Connecticut Senate 1857-1859. Later, he was elected as a Republican to the Thirty-sixth and Thirty-seventh Congresses (March 4, 1859-March 3, 1863). While in Congress, he served as chairman, Committee on Expenditures in the Department of the Treasury (Thirty-sixth Congress). He was not a candidate for renomination in 1862. After Congress, he served as judge of the Superior Court of Connecticut 1864-1875 and a Justice of the Connecticut Supreme Court 1875-1891. He moved to Hartford, Connecticut in 1892 and was Connecticut referee from 1892 until his death in a train accident near Waterbury, Connecticut in 1903. He was buried in Grove Hill Cemetery, Rockville, Connecticut.