His works often reflect a wider social context and postwar nihilism that expanded the scope and further darkened the atmosphere of the genre.

Matsumoto's work included historical novels and non-fiction, but it was his mystery and detective fiction that solidified his reputation as a writer internationally.

As a rebellious teenager, he read banned revolutionary texts as part of a political protest, which enraged Seichō's father, causing him to destroy his son's collection of literature.

Matsumoto wrote short fiction while simultaneously producing multiple novels, at one point as many as five concurrently, in the form of magazine serials.

Many of his crime stories debuted in periodicals, among them "Harikomi" (The Chase), in which a woman reunites with her fugitive lover while police close in on them.

For example at the height of the 1960 Anpo protests, Matsumoto tapped into the anti-American mood with his notorious work of "non-fiction" Black Fog over Japan (日本の黒い霧, Nihon no kuroi kiri), in which an enterprising detective uncovers a vast conspiracy by American secret agents that ties together many famous incidents and unsolved crimes of the postwar period.

[1] Likewise, many of Matsumoto's works of fiction and nonfiction revealed various aspects of home-grown corruption in the Japanese system.

In 1968 he traveled to communist Cuba as a delegate of the World Cultural Congress and ventured to North Vietnam to meet with its president later that same year.