Question: Hello, I was told [there was] an old TV show I believe something like Nick Diamond with David Janssen and Mary Tyler Moore. Is this true?
Televisionary: Well, sort of you're halfway there. The series was called Richard Diamond, Private Detective and Janssen (The Fugitive) played the titular sleuth. On the series, which ran on CBS from July 1957 to September 1959 before jumping to NBC from October 1959 to the following September, Diamond was an ex-New York cop who relocated to Hollywood.
The role of Sam, the woman who worked at Diamond's west-coast answering service, was indeed initially played by a young Mary Tyler Moore. All you ever saw of her, though, were her gorgeous legs and actress Roxanne Brooks took over the role after a short Tyler Moore run. Also in the cast: Barbara Bain (Space: 1999), who played Karen Wells, Diamond's girlfriend.