The son of Don (Jon Hamm) and Betty (January Jones) has previously been played by Maxwell Huckabee, Aaron Hart and Jared Gilmore.
Gilmore, the latest actor to play the character, has departed the AMC drama for a recurring role on ABC series Once Upon A Time.
Asked if he had any advice for his replacement, Gilmore told TV Guide: "Be careful around January [Jones]. She's not as approachable as the others."
He continued: "She's really serious about what she does. [But] everyone else is so nice."
The new actor cast as Bobby is currently expected to appear in 7 of the 13 episodes making up Mad Men's fifth season.
It was recently reported that the period drama's new $10 million (£6.1m)-per-season budget could be responsible for financial cutbacks on other AMC series, including Breaking Bad and The Walking Dead.
Mad Men will return in early 2012, with series star Jon Hamm directing the season premiere.
> Zach Galifianakis: 'I'd hate January Jones sex scene'
Watch a clip of Jared Gilmore on Mad Men below: