TMZ published a copy of the couple's pregnancy announcement, a photo of Oglesby and Johnston eating spaghetti, along with the word "we're" taped to a jar of Prego sauce.
Johnston, 24, became a household name during the contentious 2008 election, when it was announced that he was the soon-to-be father of Sarah Palin's grandchild.
He split with Bristol Palin soon after she gave birth to Tripp, now 5, in December 2008. They reunited briefly in 2010.
Johnston married Sunny Oglesby in 2012, shortly after she gave birth to their daughter Breeze Beeretta.