Unhappy with country life, he moved to Shiba in Tokyo and attended Keio University Faculty of Arts.
However, he spent more of his time in the Tokyo pleasure districts, such as Yoshiwara, rather than in the classroom. His first novel, Kotohime monogatari was based on personal experiences in the brothel district, and appeared in the literary magazine Chūōkōron in 1917. Its success led to his withdrawal from Keio University to start a career as a writer
In 1923, Muramatsu moved to China, where he lived in the Shanghai International Settlement in an apartment run by émigré Russians.
He was interested in Chinese culture, but while in Shanghai was exposed to many varieties of Western culture, due to the large numbers of French, British and Russian expatriates in his neighborhood. In his 1924 novel Mato (“Demon City”, 1924), he portrayed the dichotomy of Shanghai – a modern, beautiful, civilized façade, hiding a darker side populated by all manner of criminals and vice
Although Muramatsu produced numerous works of romantic popular fiction, he is best known for his semi-historical biographical novels. He also wrote several spy novels in the 1930s, glorifying the exploits of Yoshiko Kawashima in Manchuria.
Some of the exaggerated fictional exploits created by Muramatsu were cited as “evidence” in Kawashima’s post-war trial, which led to her execution.
Muramatsu also wrote Honchō gajin den (“Biography of an Imperial Court Painter”, 1940–1943), Kinsei Meishōden (“Biography of Contemporary Master Craftsmen”), and Kinsei meishōbu monogatari (1952–1961), which appeared serialized in newspapers. A number of novels were also made into movies in the early 1960s, such as Zangiku Monogatari (which appeared in movie form as "The Story of the Last Chrysanthemums")
Muramatsu relocated from Tokyo to Kamakura in 1947 and lived there with his mistress to his death. Muramatsu was also a noted amateur master of the classical board game, go.
He was also a noted cat lover, a passion which he shared with fellow author and Kamakura resident Jirō Osaragi.
Muramatsu died in 1961, and his grave is at the temple of Kakuon-ji in Kamakura.