The explosion also killed 24-year-old Sergeant Jason Antonio Vazquez of Chicago, whose death was reported earlier by his family.
Vazquez and Harris were part of Battery B of the 122nd Field Artillery's Second Battalion, which is based in Robbins.
The two were among about 200 Illinois Army National Guard soldiers in Afghanistan.
Earlier, the Defense Department also reported that a 19-year-old Illinois soldier died Wednesday in Iraq. Private First Class Leonard Gulczynski of Carol Stream died after a vehicle accident.
Josh Harris would have been the perfect politician. That's what his mother said. She said Harris had a contagious smile, was happy, and always had something nice to say. And he loved the military. He started saluting when he was just 9 years old, she said.
As a student, Harris learned to make choices and find purpose in life, which is the mission at Walther Lutheran High School in Melrose Park where he graduated.
"We encourage kids to have a life that has meaning, and he had a life that had meaning," said Don Gillingham, executive director of Walther Lutheran High School. "He made his decisions. He was called to that service and he was happy to go."
And after leaving Walther in 2006, he returned to visit twice.
"He stood six feet taller when he was in uniform, smiled, and had a hug for everyone and said he was ready to go over there," said Anita Ward, Walther staff member.
The reality of war has classmates asking why.
"I don't think anybody should be there right now. What's the point of sending young people and they just come home in body bags? It's not fair," said Lauren Johnson, friend and classmate.
But Gillingham said he sees Harris as an example and an opportunity for others to really measure what life is about.
"If Josh can give others the courage to accept their responsibilities, that's a real meaningful existence. That's a worthwhile person, and that's what Josh was," Gillingham said.
The IED attack also killed two coalition service members and an Afghani citizen.
Vazquez and Harris were the 17th and 18th Illinois Guardsmen killed since operations began in Afghanistan and Iraq.
The Associated Press contributed to this report.