Draper (Hamm) and Harris's (Christina Hendricks) past will be referenced in the upcoming season and Hamm joked that the pair may already have had a sexual relationship.
When asked why the womanising Draper has never had a romantic relationship with Hendrick's character, Hamm laughed: "Who says he hasn't?
"The Don/Joan dynamic is something that we do explore, and don't take this the wrong way, but we do explore as the show goes on. They have worked together for a long time, they know one another very well and I'll leave it at that."
He also spoke about the possible future between Don and Megan Calvet (Jessica Paré), arguing that the death of Ann Draper (Melinda Page Hamilton) may have caused a change in the character's love life.
"[Anna's death] may have caused some tectonic shift in Don and it may be one of the reasons we saw the proposal. It's not a mistake that he gives Megan Anna's ring, he did not give that to Betty, but I think that there is hope," he said.
Hamm was speaking at a BAFTA Q&A event at the Curzon Mayfair.
Season five of Mad Men starts on AMC on March 25. It premieres in the UK on Sky Atlantic on March 27.
> Mad Men season five to debut in UK two days after US
> Mad Men exec: New season is about change