Credit: AMC Photo: Linda Cardellini on Mad Men Season 6

Mad Men is always good for an awesome surprise mistress. 

Rosemarie DeWitt was Don Draper's (Jon Hamm) mistress on Season 1. Alexis Bledel's character had an affair with Pete Campbell on Mad Men Season 5. Julia Ormond played Megan Draper's mother, who had an affair with Roger Sterling, also on Season 5. 

Credit: NBC © NBCUniversal, Inc.; Francois Durand/Getty Images

And on last night's Mad Men Season 6 premiere, we saw that Don had started cheating on his wife Megan ... with Linda Cardellini. 

The Freaks and Geeks star, now 37, plays Sylvia Rosen, Don's neighbor and wife of his doctor buddy. At the end of the two-hour premiere, we saw Don in bed with Sylvia, whose late '60s look rendered Linda almost unrecognizable.

It's sad to see Don back to his old ways, but at least he has good taste in women!

Head over to Entertainment Weekly for a Q&A with Linda about her new homewrecking role.