Palin the daughter of 2008 vice-presidential candidate and former Alaska Gov. Sarah Palin, is featured on the cover of Us Weekly with Johnston, holding their 18-month-old son, Tripp.
The couple tells the magazine they reconnected while working out a custody plan and became engaged two weeks ago.
Palin, 19, told Us Weekly she found the idea of telling her mother about the engagement "intimidating and scary." But Sarah Palin and her husband, Todd, said in a statement on NBC's "Today" show Tuesday that they want what's best for their children and that Bristol believes in "redemption and forgiveness."
Representatives for Palin did not immediately return messages from The Associated Press seeking comment.
Earlier this month, Johnston, 20, acknowledged telling lies about the Palin family after he and Bristol broke up last year. He says he's apologized.
After the young couple broke up in late 2008, shortly after the birth of their son, Johnston gave a series of interviews criticizing the Palin family, maligning Sarah Palin's parenting skills and saying she wanted to adopt his child so people wouldn't know her daughter was pregnant.
He also posed on the February cover of Playgirl magazine sporting nothing but a sultry gaze.
Johnston told People magazine he was "unhappy and a little angry" after breaking up with his former fiancee. He said that against his better judgment, he said things about the Palins that "were not completely true."
A representative for Johnston said last month that the two were spending more time together for Tripp's sake but were not a couple.
Bristol Palin was thrust into the spotlight as a pregnant teen during her mother's unsuccessful campaign for vice president. She recently signed on with a group to offer speeches on abstinence and "pro-life" issues, among other topics, and will be paid between $15,000 and $30,000 per appearance, Palin family attorney Thomas Van Flein said in May.
Us Weekly executive editor Caroline Schaefer told NBC'S "Today" show that Bristol Palin approached the magazine looking for a platform to explain why she got engaged.
The couple is ready to get married but Palin told the magazine they'll probably see a marriage counselor, Schaefer said, adding that Palin made it clear that Levi will have "a lot of work to do."
Asked whether the magazine paid for the interview, Schaefer would not discuss details of the arrangement except to say that the magazine paid for the expenses of the photo shoot.