Educated at Daniel Stewart's College in Edinburgh, the Royal Military Academy at Sandhurst, Downing College, Cambridge and the German Armed Forces Command and Staff College in Hamburg, Melvin was commissioned into the Royal Engineers in 1975. He became Director, Land Warfare in June 2002, Director of Operational Capability at the Ministry of Defence in 2004 and General Officer Commanding United Kingdom Support Command (Germany) in 2006. He went on to be Chief Army Instructor at the Royal College of Defence Studies in 2009 before retiring in 2011.
In 2009 he appeared as an expert witness at the International Criminal Tribunal for Yugoslavia. He is an associate senior fellow of the Royal United Services Institute.
During his Army service in Germany, Melvin learned German and developed an interest in German military history. The product of this was his 2010 biography of Erich von Manstein. Manstein is widely regarded as the most gifted German commander of World War II, but he was also a convicted war criminal who never acknowledged his own or the German Army's responsibility for the crimes committed on the Eastern Front while he held major commands there. In his prologue to the book, Melvin writes:
"One is compelled to ask why such an eminent master of the profession of arms acted as a willing servant to Hitler for so long in pursuit of a manifestly unjust war, in which so many heinous crimes were committed by Germany's armed forces."
Melvin's conclusion was that Manstein was a product of his age, his class, his education and his own stubborn personality, all of which blinded him to the ethical conflict between his duty as a German officer to obey the orders of the legitimate government, and the increasingly criminal nature of the Nazi regime.
"He neither questioned the inhumane manner in which the German campaign in the East was being conducted nor anticipated its devastating outcome. He was hardly alone in this matter."
Reviews of Melvin's book concentrated on this question. Alexander Rose in the New York Times referred to "Mungo Melvin’s authoritative and splendidly comprehensive biography", and called Manstein
"a soldier with a spine ramrod straight, just no backbone at crucial times... Manstein's fatal vanity lay in always believing that he could distinguish between the welfare of his beloved army and Hitler’s demands."
Rose was also critical of what he saw as Melvin's narrow focus on military matters. He wrote:
"Inspirational, absolutely, but all this is just a gumbo of dated historiography, hero-genius worship, the 19th-century Great Man Theory of History... The biographical art has transformed itself over the last century to encompass a host of fresh approaches — thematic, psychological, cultural, among others — yet military biography, especially as it pertains to the great captains, remains mired in Victorian conventions. While “Manstein” is a superb work in many respects, certainly a lasting monument to patient excavation of the archives, it suffers from failing to rise above the confines of its creaky genre."
Tom Nagorski in the Wall Street Journal called Manstein "interesting but flawed," finding fault with its concentration on detailed descriptions of Manstein's work as a military commander. He wrote:
"The author is a retired British general, and while his fellow officers may value the book's density of detail, the ordinary reader may find it hard-going at times. This is history told through maneuvers—encirclements and tank formations and the like. The pathos of battle is in short supply. Even Stalingrad—the scene of so much drama, bravery and horror—is presented largely as a tale of strategy and tactics."