Rosendo Xo Pec is accused of killing Willis in Santa Ana as he was attempting to rape her, officials said.
Willis recorded songs that included "Someday You'll Need My Love," ''Act Naturally" and "Take My Heart" in the 1960s.
She worked with a number of prominent musicians, such as Bill Medley of the Righteous Brothers and pianist/songwriter Leon Russell, her friend Anthony Reichardt wrote in a Facebook tribute.
Some of her recordings can still be found on YouTube:
After her singing career ended, Willis worked for many years at the Santa Ana post office before retiring.
It appeared she had fallen on hard times later in life, as police say she was homeless at the time of the fatal assault on New Year's Day.
Officials say Willis was sleeping in a strip mall parking lot around 4:15 a.m. on Jan. 1 on First Street in Santa Ana when Pec approached her.
He allegedly removed her blanket and sexually assaulted her, according to the Orange County District Attorney's Office. As she began to scream for help, Pec allegedly punched her in the head multiple times and choked her to keep her quiet.
Someone called 911 and officers say when they arrived, Pec had his pants down and was on top of the victim. He tried to run away, but officers were able to detain him near the scene.
Willis was pronounced dead at the scene.
Pec has been charged with murder with the enhancement of being committed during a rape. He faces a possible sentence of life in prison without possibility of parole.
His arraignment had been scheduled for Wednesday, but was postponed to Jan. 19.