Johnston, 20, is the father of the younger Palin's 18-month-old son, Tripp. The couple called off a previous engagement shortly after the child was born in December 2008.
"We got engaged two weeks ago," Palin, 19, told Us Weekly. "It felt right, even though we don't have the approval of our parents."
"I really thought we were over," Johnston added. "So when I went, I had no hope. I think we both just started talking -- and then we took Tripp for a walk."
"When he left that night, we didn't hug or kiss, but I was thinking how different it was. He texted me: 'I miss you. I love you. I want to be with you again' ... I was in shock," Palin chimed in.
As for what her mother will say about their betrothal, Palin acknowledged to the magazine: "It is intimidating and scary just to think about what her reaction is going to be. Hopefully she will jump on board."