At the outbreak of the civil war in 1861 he returned to the North. From 1862 to 1864 Sweet worked as mechanical draftsman in England, and back in the States was engaged in bridge building. He also made some inventions. At the Paris Exhibition of 1867 he had made quite an impression with the introduction of his linotype machine.
From 1873 to 1878 het was Professor of practical mechanics at the Sibley College of Mechanic Arts of the Cornell University, where he also headed the Sibley College. Afterwards he was founding President of straight line engine works. At the World's Columbian Exposition in 1893 he was expert for the government, and one of the jurors on machine tools.
In 1880 Sweet was a key founder of the American Society of Mechanical Engineers (ASME), and its third President in the year 1884–85. In 1914 the ASME awarded him the John Fritz Medal "for his achievements in machine design, and for his pioneer work in applying sound engineering principles to the construction and development of the high-speed steam engine." In 1914 he also received the honorary degree of Doctor of Engineering from the Syracuse University.