Ron Barber, 65, has spent the past six months undergoing extensive physical therapy and trying to cope with the trauma of the shooting and loss of friends.
He was diagnosed with post-traumatic stress disorder and has been haunted by nightmares and vivid memories of that day.
Barber was shot in the cheek and thigh during the Jan. 8 attack that killed six people and injured 13, including Barber and Giffords. The shooting killed his colleague, Gabe Zimmerman, and John Roll, Barber's college buddy and a federal judge.
"I'll never forget the congresswoman being shot. I'll never forget Gabe dying beside me," Barber told The Associated Press. "John was standing beside me and I was the last person to speak to him ... Those are the memories that were with me when I woke up this morning. Those are the thoughts that just keep going through my head."
The tearful Barber, still walking with a cane, returned to his job as district director at Giffords' Tucson office on a part-time basis as he continues rehab and dealing with fatigue, pain and sorrow. He walked in slowly with his wife Nancy by his side as a crowd of co-workers cheered.
The staff showed up early at the office to put up big yellow balloons with smiley faces and a giant, homemade sign that read, "Welcome Back Ron!"
"Good morning! What's been happening the past six months?" Barber joked before hugging a dozen people. "It's like my first day at school."
Barber later said he had been trying to get his doctors to allow him to go back to work for the past two months and only recently got final permission.
"I'm still a little apprehensive about what I can do," he said. "I don't know how else I'm going to find out if I can do this if I don't try."
The gunshot that hit Barber's cheek exited the back of his neck and barely missed his spinal column, while the shot that struck his thigh caused him more serious medical problems and has hindered his ability to walk. He has limited feeling in his lower left leg.
Doctors said Barber likely would have died without the help of a bystander, Anna Ballis, who applied pressure to his thigh wound. Police also credited Roll with helping save Barber's life.
Surveillance video of the shooting shows Roll pushing Barber to the ground, helping him crawl under a table and then lying on top of him.
Jared Lee Loughner, 22, has pleaded not guilty to charges stemming from the shooting. He is being held at a Springfield, Mo., facility undergoing treatment that is meant to make him mentally competent to stand trial.
An appeals court ruled late Friday that prison officials must temporarily stop forcing anti-psychotic drugs on Loughner while it considers arguments from prosecutors and the defense on whether the drugs should be allowed. Prison doctors concluded that Loughner was a danger and needed to be medicated.
Another Giffords' staffer, Pam Simon, was wounded in the shooting after taking a bullet to the chest and one to the wrist. She returned to work in February.
Simon told the AP that having Barber back was another milestone for the office staff as everyone continues to move on from the shooting.
"It makes it feel a little more normal," she said. "Ron was always the captain of the ship, and now it feels like we're back on course."
Barber's first order of business was to set up one-on-one meetings with office staffers to catch up on business and get back on track even more.
"These are all milestones," Barber said. "The next one will be when Gabby gets back."