 Prior to his return in September 1868, the Democratic Party had expressed some interest in nominating him for president again, but Ulysses S. Grant became the Republican candidate in May 1868, and this interest died.McClellan worked on engineering projects in New York City and was offered the position of president of the newly formed University of California, which he declined..

McClellan was appointed chief engineer of the New York City Department of Docks in 1870.Beginning in 1872, he also served as the president of the Atlantic and Great Western Railroad.