Harris, 35, was accused of intentionally locking Cooper inside a hot car for seven hours. On that same day, Harris was sexting with six women, including one minor, according to phone records.
In addition to three counts of murder, Harris was found guilty of two counts of cruelty to children for Cooper's death, and guilty of three counts relating to his electronic exchanges of lewd material with two underage girls.
"This is one of those occasions where actions speak louder than words," Cobb County Assistant District Attorney Chuck Boring said after the verdict. "He has malice in his heart, absolutely."
The trial, which spanned almost five weeks, was moved to the Georgia coastal town of Brunswick from Cobb County, outside Atlanta, after intense pretrial publicity. It was briefly interrupted by Hurricane Matthew.
The Glynn County jury of six men and six women deliberated for 21 hours over four days. Jurors considered the testimony of 70 witnesses and 1,150 pieces of evidence, including the Hyundai Tucson in which Cooper died in a suburban Atlanta parking lot.
Harris showed little reaction as the verdict was read in court.
Cobb County prosecutors argued that Harris intentionally locked Cooper inside his car on a hot summer 2014 day because he wanted to be free of his family responsibilities. Harris' lawyers claimed the boy's death was a tragic accident brought about by a lapse in memory.
It was June 18, 2014, when Harris, then 33, strapped his son into a rear-facing car seat and drove from their Marietta, Georgia, home to Chick-fil-A for breakfast, then to The Home Depot corporate headquarters, where he worked. Instead of dropping Cooper off at day care, testimony revealed Harris left him in the car all day while he was at work.
Sometime after 4 p.m. that day, as Harris drove to a nearby theater to see a movie, he noticed his son was still in the car. He pulled into a shopping center parking lot and pulled Cooper's lifeless body from the SUV. Witnesses said he appeared distraught and was screaming.
"'I love my son and all, but we both need escapes.' Those words were uttered 10 minutes before this defendant, with a selfish abandon and malignant heart, did exactly that," said Boring in his closing argument.
The prosecution argued that Harris could see his son sitting in his car seat in the SUV.
"If this child was visible in that car that is not a failure in memory systems," Boring argued. "Cooper would have been visible to anyone inside that car. Flat out."
If Cooper was visible, Boring said, "the defendant is guilty of all counts."
After the verdict, jurors told the prosecution that the evidence weighed heavily in their decision, Boring said.
Digital evidence showed that on the day his son died, Harris exchanged sexual messages and photos with six women, including one minor.
State witnesses testified that Harris lived what prosecutors described as a "double life." To his wife, family, friends and co-workers, Harris was seen as a loving father and husband. But unbeknownst to them, Harris engaged in online sexual communication with multiple women, including two underage girls, had extramarital sexual encounters in public places and paid for sex with a prostitute.
Harris' defense maintained that his sexual behavior had nothing to do with Cooper's death.
"The state wants to bury him in this filth and dirt of his own making, so that you will believe he is so immoral, he is so reprehensible that he can do exactly this," said defense attorney H. Maddox Kilgore during his closing argument.
Kilgore argued that Cobb County police investigators focused only on matters that fit the state's theory and ignored all the evidence that pointed to an accident.
"You have been misled throughout this trial," Kilgore told jurors.
The defense lawyer continued to maintain his client's innocence after the verdict. He said he plans to appeal the verdict.
"When an innocent person is convicted there's been some breakdowns in the system and that's what happened here," Kilgore told reporters outside the courthouse.
"From the moment we met Ross Harris we've never, ever once wavered in our absolute belief that he is not guilty of what he's just been convicted of."
The main defense witness: Harris' ex-wife
The defense's key witness was Harris' ex-wife and Cooper's mother, Leanna Taylor.
"Cooper was the sweetest little boy. He had so much life in him. He was everything to me," Taylor recalled, as she seemed to fight through tears.
For two days, Taylor told jurors private details of her married life with Harris, saying they had intimacy problems and recounting Harris' struggles with pornography.
Marital struggles aside, Taylor described Harris as a "very involved" parent who loved their son. In her mind, she said, the only possible explanation was that Harris "forgot" Cooper and accidentally left him in the car.
Boring said it did not matter that Taylor declined to speak with the prosecutor's office and testified for the defense. "As far as proving the case we did not need her," he told CNN.
Harris is expected to be sentenced December 5. He could face life without parole, though Boring said the prosecution will speak with the family to determine what kind of sentence to ask for.