In a case that has attracted national attention, a jury found David Westerfield guilty on all charges after deliberating for more than a week.
Danielle van Dam disappeared from her San Diego home on 1 February. Her body was found in a rural area about 25 miles from her home on 27 February.
Westerfield, a 50-year-old twice-divorced father of two, was later arrested after a police search of his home found child pornography.
The six-man, six-woman jury found Westerfield guilty of murder, kidnapping and possessing child pornography, and now has to decide on his future.
He will learn on 28 August whether he will receive the death penalty or life in prison without the possibility of parole.
Publicity
The case received national press coverage, in part because of the allegations made by Westerfield's lawyers about the lifestyle of the murdered girl's parents.
It was also the first of six other widely-publicised abductions of young girls across the United States this year.
Defence lawyers had argued that the van Dams' sexual activity with other partners, and their use of marijuana, exposed their daughter to danger.
They suggested that someone other than Westerfield had kidnapped her.
Prosecutors focused on forensic evidence - including a single drop of Danielle's blood that was discovered on Westerfield's jacket - and ridiculed the accused's alibi.
Westerfield told police that he spent the weekend driving, alone, across the vast California desert.
Legal experts expect Westerfield to appeal against his conviction.