Porter performed with distinction during the Civil War, receiving the Congressional Medal of Honor for his action at Chickamauga. In April 1864 he was appointed aide-de-camp to Grant, and in March 1865 was brevetted brigadier-general. He remained a trusted friend and advisor of Grant for many years, serving Grant as military secretary during the first part of Grant's presidency. Resigning in 1872, he became a prominent railroad executive by joining the Pullman Company, promoting elevated railway lines in New York City, and becoming president of the New York, West Shore, & Buffalo Railway. He served as ambassador to France 1897-1905, and was singularly responsible for locating the body of John Paul Jones in France and having it transported back to the United States for reburial. During his lifetime he received numerous honors from foreign governments such as the Grand Cross of Legion of Honor (France) and the Gold Medal for Patriotism (Turkey).