She joined the department in 1931 and by 1939 was the fourth woman to achieve the rank of first-grade detective in the NYPD.
She was a minor celebrity in New York City news, appearing in articles chasing down and arresting thieves and pickpockets.
[6] Of her, The New York Times wrote in 1938: In more than seven years on the police force Miss Shanley has had considerable experience with man-catching.
"[7]In 1941, Shanley shot her gun while she was off-duty and intoxicated at a bar in Jackson Heights, Queens.
In 2016, actress Rachel McPhee put on a one-woman show devoted to her, Dead Shot Mary, at the Bridge Theater in New York City.