[2] Upon the outbreak of the Civil War, Zimmerman left the academy and tried to join the Union Navy, but was refused because he was too young.

He participated in the Battle of Saint Charles and Duvall's Bluffs and served with distinction during the capture of Arkansas Post in 1863, after which he was promoted to Ensign.

During the campaign, he was involved in several missions and was promoted to acting Master and Officer of the side wheel steamer USS Ouachita after the surrender of Vicksburg which he held until the end of the War.

[3] During the remainder of the war, he was eventually made a Lieutenant and a ship commander at the age of nineteen (the youngest officer at his rank in the entire U.S. Navy).

The Detroit, Toledo & Ironton Railway had taken control of the Ann Arbor Railroad from Rudolph Kleybolte & Co. in June 1905.

The DT&I went bankrupt in 1908,[12] forcing them to divest Ann Arbor Railroad to Zimmerman, who sold the line in 1909 to Joseph Ramsey, Jr. and Newman Erb,[13] and retired from active business while still retaining control of his "immense coal and iron lands" in the Midwest.

[15] At the time of his death, he was preparing to testify before Commissioner Henry Clay Hall of the Interstate Commerce Commission regarding the sale of the Cincinnati, Hamilton and Dayton Railroad and the Père Marquette to J.P. Morgan & Co.,[1] for which Zimmerman made a profit of more than $1,000,000.

"[16] Frederick W. Stevens testified that Zimmerman and his associates loaded $24,000,000 worth of obligations on the railroad and took a 999-year lease on the Père Marquette system and guaranteed the road's bonds.

"Etta" Evans (Urbana, Champaign County, Ohio, 1848 – 23 December 1882), a daughter of Abraham Evans of West Liberty, and wife (Champaign County, Ohio, 17 March 1845) Elizabeth Igou, daughter of Peter Igou, farmer, and wife Susan McKenzie.

[19] Before her death in December 1882 from peritonitis,[20] they were the parents of one child together: Helena Zimmerman (1878–1971), who married William Montagu, 9th Duke of Manchester in 1900.

A few minutes before his death, he had been laughing over a $100,000 breach of promise suit filed against him in New York by Icy Wareham the previous June.