The 38-year-old actress and Penn, 53, started dating in 2013 and Theron feels she found love when she was not looking for it, reported Esquire magazine.
"I wasn't in any place that I even wanted something like (a relationship). Then one day you go: Oh, (my son Jackson's) almost two now... And you have more time again.
"And it's like if you are open to something, if you just let it happen, it will happen. When you least expect it," Theron said.
"It was nice to be single and now it's nice to be not single," she added.