After his forced retirement, Flynn went into business for himself.
He founded a New York detective agency with his daughter Veronica and son Elmer as partners, which generated some income.
However, according to Dash, the partnership was unsuccessful and contributed to the business's ultimate demise: Veronica and Elmer "were running the detective business into the ground.
Both heavy drinkers, they overspent and upset clients.
The pair's increasingly erratic behavior distressed their more abstemious father, and the worry weakened him."

Flynn earned the bulk of his income, at that point, from writing.
He had occasionally contributed articles, typically about his greatest cases, to such journals as The New York Herald and The Washington Post since 1911.
After retiring, he worked briefly as a crime novelist.
Flynn also became a scenario writer for the motion picture industry through his acquaintance with the actor King Baggot who, Dash notes, was considered the greatest film star in the country at that time in 1912.
The producers Theodore and Leopold Wharton commissioned him to write story lines for their films, including The Perils of Pauline, and eventually adapted Flynn's experiences into a 20-part spy thriller titled The Eagle's Eye (1918), starring Baggot.
The same year they were also published as weekly installments in The Atlanta Constitution's magazine section under the title The Eagle's Eye: True Story of the Imperial German Government's Spies and Intrigues in America.
Some of these episodes were published in a book with the same name in 1919.

He also edited a magazine which bore his name, Flynn's Weekly Detective Fiction, and became the longest-running, most successful journal of its genre.
After Flynn's death, the periodical was temporarily renamed Detective Fiction Weekly (formerly Flynn's) before resuming its original title; the periodical published a total of 703 issues.
After his forced retirement, Flynn went into business for himself. He founded a New York detective agency with his daughter Veronica and son Elmer as partners, which generated some income. However, according to Dash, the partnership was unsuccessful and contributed to the business's ultimate demise: Veronica and Elmer "were running the detective business into the ground. Both heavy drinkers, they overspent and upset clients. The pair's increasingly erratic behavior distressed their more abstemious father, and the worry weakened him."
Flynn earned the bulk of his income, at that point, from writing. He had occasionally contributed articles, typically about his greatest cases, to such journals as The New York Herald and The Washington Post since 1911. After retiring, he worked briefly as a crime novelist. Flynn also became a scenario writer for the motion picture industry through his acquaintance with the actor King Baggot who, Dash notes, was considered the greatest film star in the country at that time in 1912. The producers Theodore and Leopold Wharton commissioned him to write story lines for their films, including The Perils of Pauline, and eventually adapted Flynn's experiences into a 20-part spy thriller titled The Eagle's Eye (1918), starring Baggot. The same year they were also published as weekly installments in The Atlanta Constitution's magazine section under the title The Eagle's Eye: True Story of the Imperial German Government's Spies and Intrigues in America. Some of these episodes were published in a book with the same name in 1919.
He also edited a magazine which bore his name, Flynn's Weekly Detective Fiction, and became the longest-running, most successful journal of its genre. After Flynn's death, the periodical was temporarily renamed Detective Fiction Weekly (formerly Flynn's) before resuming its original title; the periodical published a total of 703 issues.
William J. Flynn