Walsh began writing as a reporter for the Baltimore Sun and for the U.S. Army Historical Branch, but by 1933 he retired from journalism and turned to mystery writing. As a self-employed writer for thirty-six years, Walsh published more than fifty short stories and eleven novels--each set in the streets of New York City and depicting various elements of the city's vital population. His 1955 Nightmare in Manhattan, described as maintaining a consistent level of suspense throughout, received an Edgar Award for best first mystery and later became the successful film "Union Station", starring William Holden. Walsh's other novels include, Night Watch, Dark Window, Dangerous Passenger, The Action of the Tiger, and The Eye of the Needle, a 1961 Inner Sanctum Mystery Contest winner.