Smyth served as one of the board of managers of the National Homes for Disabled Soldiers. He was a delegate-at-large to the 1872 Republican national convention, and President Hayes appointed him honorary commissioner to the 1878 International Exposition at Paris. Smyth was a principal stockholder and president of the Concord and Montreal Railroad. He was a trustee of the New England Conservatory of Music in Boston, and served as president of the New Hampshire Orphans' Home at Franklin.