Question is, did Doyle become a detective after writing one, or was he a detective first?

It was one of the most sensational crimes—and most scandalous wrongful convictions—of the 20th century, a case that would be known as the Scottish Dreyfus affair. It involved a savage murder, stolen jewels, an international manhunt and a wily maidservant who went to her grave knowing far more about the killing than she would ever disclose.Even more remarkably, it involved the world’s foremost writer of detective fiction, playing real-life detective on a case in which the stakes could scarcely be higher—a case, he wrote, that was a “disgraceful frame-up, in which stupidity and dishonesty played and equal part.”

Source: The Time Arthur Conan Doyle Helped Solve a Real Murder Case