Walter Mashburn appears first in Redline. He is a billionaire, with a penchant for vintage luxury cars and beautiful women.
He asks Teresa Lisbon out, but she refuses him. He has a great ability in understanding people and defines himself as a man who "hate being ignored (...), cunning, ruthless" and who "hate to lose". According Jane, he has a difficult relationship with his mother. He has had many wives, at least three or, as he puts it, "enough to know better"; his third wife was interested in the paranormal, so he recognizes immediately that Patrick Jane is a false psychic. Mashburn has tried everything, is bored, and will do anything for a new thrill. As a boy, he ended up in reformatory because of his behavior in school. There he learned how to make bombs and firecrackers. Around 1997, Walter founded Mashburn Avionics, a company of electronic components (especially in the avionic area) and hi-tech weapons. By selling it and also acquiring other companies, he became extremely wealthy. He has a large mansion in Marin County, a castle in Scotland, and other residences and properties as well as many cars, boats, helicopters and a private jet that he pilots himself.When Mashburn reappears in season 3 (Red Hot), he and Lisbon have a one-night stand. Walter asks Teresa when they will meet again, but Teresa reminds him that he is going to travel to Europe on business and will be gone for two months. She says that they will see what happens when he returns.
He married his several wives after a great love delusion, who "broke something in him", when is left by his fiance, Marie Jarrett, shortly before the wedding; she then married Yuri Bajoran, a former KGB agent and future billionaire enemy of Walter. When Walter financially beats Yuri and buys his helicopter company, Marie and Yuri trying to frame him for the fake murder of Bajoran, and then try to kill him. They end up in prison thanks to Patrick Jane and CBI. His name is in Jane's List of Red John Suspects in Black Cherry and in list of 30 suspects on CBS website, but does not appear on the final list of suspects in Red John's Rules .