Her nude body was found near a park barbecue pit.
The youngster had skipped school a day before to spend time with her boyfriend and was last seen leaving his house to catch a bus home.
Former security guard Hughes, was also accused and found guilty of murdering two prostitutes, Verna Williams and Deborah Jackson, in 1986 and 1993.
According to the LA Times Deputy District Attorney Beth Silverman showed jurors a slideshow to illustrate how the victims' bodies were posed in sexual ways to shock anyone who found them.
The 55-year-old was already serving a life sentence for killing four women in 1992 and 1993 when he was found guilty for these additional murders.
Cops said DNA linked him to the gruesome crimes.
Prosecutors will now seek the death penalty when the sentencing trial begins on Monday.
Hughes was investigated by a task force looking into three separate serial killers accused of stalking women during a crack cocaine epidemic that forced women onto the streets as prostitutes to feed their addictions.
As many as 90 women were killed in inner-city Los Angeles during that time.