Earlier this month a judge found 36-year-old Robert O'Ryan guilty of stalking Johnson after police uncovered what appeared to be a potentially deadly plot to kidnap the sports star in March 2009.
Even though it terrified her, Johnson said she took the stand and faced her stalker because she said she "had to."
"I didn't want to. I kept asking if I could stay in the back room or go home," Johnson told "Good Morning America" in an exclusive interview. "I had to stand up for everybody else out there that's been through it... I just had to go there and kind of help put him away."
For much more on this story tune in to "Nightline" tonight at 11:35 p.m. EST.
Earlier this month Los Angeles Superior Court Judge Michael Pastor found O'Ryan guilty of felony stalking and burglary charges and two misdemeanor concealed weapons violations. The verdict came after four days of testimony and evidence.
O'Ryan was arrested on March 23, 2009, after driving from Florida and allegedly jumping over a fence at the television studio where Johnson was a contestant for "Dancing With the Stars."
Police searched O'Ryan's car and found two loaded guns, duct tape, a wooden club and handwritten poetry by O'Ryan to Johnson. In one passage, O'Ryan wrote, "No matter what happens I will always love you."
"Before all this happened, I thought I was invincible," Johnson told "GMA." "I thought nothing like this could happen to me and nothing would ever happen. But it was a reality check. It made me change the way I go about everyday life.... You hate to say, it's real."
O'Ryan had pleaded not guilty by reason of insanity. Despite the guilty verdict, O'Ryan was ordered to go through a mental evaluation.
He could get close to six years in state prison if he is found to be sane or, potentially, the rest of his life in a mental hospital if he is found insane.
Johnson Faces Alleged Stalker in Court
Johnson came face-to-face with O'Ryan in the courtroom, and testified about how frightened she was by the ordeal.
Prosecutor Wendy Segal asked the 18-year-old Johnson if she knew O'Ryan and how she felt after reading some of the poetry.
"I didn't know him. He was a stranger to me. To see someone with that kind of obsession made me really scared," Johnson said.
Johnson -- who said she was "very nervous" on the stand -- testified that she considered quitting "Dancing With the Stars" early and going home to Iowa after the incident.
"I heard he'd said he was coming to L.A. to marry me, that I was the mother of his unborn children," Johnson said.
Johnson and her parents eventually decided to continue on "Dancing with the Stars" and instead altered her normal routine -- changing vehicles, hotels and cancelling all personal appearances.
Shawn Johnson's Stalker Stopped by Cop
During the trial O'Ryan's attorney tried to demonstrate that O'Ryan cooperated with the police investigation by telling investigators where he had hidden weapons in his car and by providing them with the combination to a safe that contained gifts for Johnson -- a picture frame and a small wooden purse.
Five days before his arrest O'Ryan had been pulled over in Alabama, and the dash-cam video of the traffic stop recorded him telling the police officer that he intended to "try to get her [Johnson] to marry" him.
"I know if sounds a little bit crazy, but my intuition tells me that we're gonna have a beautiful relationship together," O'Ryan told the policeman.
During the 29-minute stop, O'Ryan mentioned Johnson at least three times.
When the policeman asked O'Ryan where he was headed, O'Ryan said "California" and identified Johnson as "an Olympic gymnast. ... She's on TV now."
"I'm meeting her there and I'm going to try to get her to marry me," he said.
The Alabama policeman issued O'Ryan two tickets, one for not having insurance and the other for an expired license plate.
"They let these guys go. I don't know why," Johnson said. "It just makes you want to be a little mre cautious than you think you should be."
O'Ryan was an Olympic hopeful before an injury caused severe depression, a friend told ABC News. He graduated from the University of Florida with a degree in engineering and tried out for the Olympic rowing team, but hurt his back, the friend said.
Shawn Johnson's Message to Targets of Harassment
Johnson said one of the reasons she wanted to appear on television for an interview was to give advice to anyone that's been in a similar situation.
"Stay strong. Stand up. Have a voice," she said. "Tell people what you're feeling, how you're feeling and you can never be too safe. It's better safe than sorry."
ABC News' Monica Escobedo, Lee Ferran, Katie Escherich and Ned Potter contributed to this story. Additional information from The Associated Press.