At the time, Sarah Palin announced that the parents-to-be planned to marry. She said that having a baby would make her daughter "grow up faster than we had ever planned."
Then Johnston showed up alongside Bristol Palin at the September 2008 Republican National Convention.
He quickly found the intensity of the political spotlight focused on every aspect of his life.
He came under early scrutiny when a MySpace page emerged on which Johnston was quoted as saying "I'm a f-----' redneck" and "I don't want kids." Johnston said that his friends created the page and that he had nothing to do with it.
But Johnston said all the extra attention he received from being in the center of a presidential race didn't ruin his life.
"I don't — that kind of came with dating Bristol. I mean, that came with the package, I suppose. You gotta know how to deal with it. But no, nothing has changed. I mean, nothing out of the ordinary I guess," said Johnston, who added he had no regrets.
"We've been dating for a long time, and I wouldn't take it back," he said.
Despite reports from tabloid publication Star Magazine last week that alleged Bristol Palin doesn't allow Johnston to see his infant son, Tripp, the teen father said he sees his son regularly and described how becoming a father had changed his life.
"I don't know where they got that from, but I can go over there whenever and she lets me take him. We're still really good friends. I mean, none of that was true," Johnston said.
"I'd give anything for [Tripp], to be with him," he added. "[There are] a lot of changes when you're a father, when you hold him for the first time, you know. I don't do a lot of things I used to anymore, I'll tell you that."
For now Johnston said he's concentrated on getting his life together and getting a job.
"Yeah, [I'm] growing up a lot," said Johnston, whose only images of his son are ultrasound pictures. "So, it's fun. It's good times."
Bristol Palin told Van Susteren that no one had pressured her to have the child.
"It was my choice to have the baby," the 18-year-old said. "It doesn't matter what my mom's views are on it. It was my decision."
While campaigning for governor of Alaska in 2006, Sarah Palin came out strongly against abortion and said that abstinence should be taught as part of sex education in public schools. Her decision to give birth to her youngest son, Trig, who was prenatally diagnosed with Down syndrome, has been held up by conservatives as exemplary of anti-abortion values.
Sarah Palin also discussed abstinence and teen pregnancy in the interview with Van Susteren.
"You get behind that, that ideal of, yes, abstinence, you know?" the new grandmother, 44, said. "Hey, don't get pregnant. Well, get beyond that when it happens, and then you deal with it. Life happens. Life happens and you deal with it, and Bristol's dealing with it wonderfully."
"Not the most ideal situation," she added. "Certainly you make the most of it."
In October, Johnston told The Associated Press he had been dating Bristol Palin since freshman year of high school and the couple had long planned to get married.
"We were planning on getting married a long time ago, with or without the kid," Johnston told the AP. "That was the plan from the start."
He described rough wedding plans, even naming two friends he had asked to be groomsmen.