Harris is accused of murdering 22-month-old Cooper, who was pronounced dead on June 18, 2014, after spending about seven hours in a car seat in Harris' locked SUV in the Atlanta area. That day, temperatures there were in the low 90s.
Authorities say Cooper was in the car when Harris drove to work at a Home Depot corporate office that morning. When Harris went inside, Cooper was left in the vehicle. Harris returned to his car during lunch to put something away, then went back to work.
Testifying in defense of her ex-husband at his trial in Brunswick, Georgia, today, Taylor recounted how she learned from her son's day care teachers that her husband had not taken Cooper in that day.
"We were trying to figure out when Ross left work," she said. She and Harris were married at the time of Cooper's death. "One of the day care teachers called me and said, 'Leanna, I think it's time to call the police.'"
"I was in complete denial about the situation," Taylor said. "My phone rang, and it was a detective ... I said, 'Will you please just tell me what's wrong? I know something's wrong.' They said, 'No, we're going to talk about it when we get there.' I said, 'It's bad, isn't it?' And he said, 'Yeah, it's bad.'"
When detectives arrived, she said she felt the situation wasn't real.
"They told me that my son was deceased," Taylor said. "I didn't know what had happened. I didn't know if Ross was OK. I didn't know anything."
"I needed more information," Taylor said. "It just wasn't real. It was like I wasn't even there. I couldn't understand."
"Sometimes it still doesn't feel real," she said, crying.
Taylor said that when Harris was questioned by police, he was "broken and just beside himself," she said.
Taylor described Harris as a good father who was hands-on and enjoyed time with his son.
"Based on everything that I knew that day, Ross must have left him in the car. That was the only thing that made sense. The only thing that clicked in my mind as even a remote possibility," she said. "He must have forgot."
Court documents state that Harris allegedly researched child deaths in hot cars before the incident.
Charges in the indictment refer to sexually explicit online exchanges from March 2014 through the day of Cooper's death that, prosecutors say, Harris had with an underage girl. Prosecutors argued that Harris wanted to be free of his family responsibilities and was having multiple online affairs, including with the girl.
Defense attorneys say Cooper's death was an accident and that Harris forgot his son was in the car.
Taylor testified today that she and Harris had intimacy problems; she said that she wanted to make their marriage work and that they went to counselors alone and together.
During cross-examination, prosecutors asked her about her relationship with Harris.
She said that during their marriage, decision-making was done together. She described him as "kind of go with the flow" and herself as "a little more structured."
Taylor said she didn't know the depth of what prosecutors described as his double life, adding that his behavior at home was loving.
She said years before Cooper's death, Harris confessed to her that he had a pornography problem. Later, she said, she saw he had sent messages to another woman, asking for photographs. The messages made her angry, Taylor said.
She testified that despite their issues, she didn't think at the time that Harris would have a physical affair.
"I knew that we had problems in our relationship ... but I didn't know what he was doing," she said. "I didn't think that he would cheat on me."
She added that Harris had "a part of himself that he was not sharing with me."
Harris faces eight charges: malice murder, two counts of felony murder, cruelty to children in the first degree, cruelty to children in the second degree, criminal attempt to commit a felony and two counts of dissemination of harmful material to minors. He has pleaded not guilty to all the charges.
ABC News' Toria Tolley contributed to this report.