Police said Cunningham was working as a security guard at an apartment complex on Memphis' southeast side, when he heard a gunshot at the neighboring Cherry Crest apartments and went to investigate about 2 a.m.
Police didn't provide details, but they said when officers arrived, both the 70-year-old Cunningham and a 16-year-old boy were found dead from gunshot wounds.
The teenager has not been identified. Calls to police by The Associated Press were not returned.
When contacted at his home in Washington D.C., Bill Cunningham confirmed his brother was killed.
Byny Garcia, who lives in a unit across the courtyard from where the shooting took place, said Cunningham was well-liked.
"He'd take care of the old people and the Spanish kids," Garcia said. "He was a good person. We don't feel good at this moment."
Born Blake Baker Cunningham Jr., the keyboardist and singer established a national reputation in 1965 as a member of the touring version of Ronnie and the Daytonas, known for the song "G.T.O."
That band eventually became the Hombres, which scored a chart hit with "Let It All Hang Out" in 1967.
After the Hombres' career slowed, Cunningham went to work behind the scenes at the famed Sounds of Memphis Studios. In 1971, he moved to Los Angeles where he served as chief engineer at Independent Recorders, working with the likes of Billy Joel, Elton John, and Lou Rawls.
Cunningham ultimately returned to Memphis a few years later and launched his own studio.
He had been a member of Lewis' band since 1997. His solo album, "Hangin' In," was released in 2003.