Armstrong will begin his role as the hospital's director of security Monday, Feb. 1. He'll stay with the city until January 31 and in that time he'll assist Mayor Jim Strickland in selecting a new police director.
Strickland said he will now being a national search for a permanent replacement.
Armstrong enrolled in the city's Deferred Retirement Option Program [DROP] last year and was scheduled for retirement in 2017.
"It is not surprising that one of the crown jewels of our city, St. Jude, would recognize the talent and leadership of our city's police director, Toney Armstrong," Strickland said in a statement. "I sincerely thank Toney for his commitment and dedication to the city, and for his work with the Memphis Police Department."
Armstrong served the MPD for 27 years. He returned to Memphis after serving in the Army and began his work with the department. He worked undercover in the Organized Crime Unit and served in the homicide division for five years.
When former Mayor A C Wharton named him director in 2011, Armstrong was the youngest person to serve in that position.
"I am extremely grateful to have served the city in this capacity," Armstrong said in a statement. "Because I wanted to stay in Memphis, I searched for a potion that was purposeful and as meaningful as law enforcement has been to me and St. jude was the answer."
Memphis Police Director [MPD] Toney Armstrong will leave the department next month for a job at St. Jude Children's Research Hospital, according to a Thursday morning statement from Memphis City Hall.