1
general::
Also, in the saddle. In control, in a position of authority. For example, With the boss on vacation, Mr. Burns was in the driver's seat and enjoying it, or She waited until after the election, knowing that she'd be in the saddle then. The first expression dates from the 1800s, the second from the early 1600s. Also see AT THE HELM.
American Heritage Idioms
2
general::
Phrase(s): in the driver’s seat
Fig. in control; in charge of things. (As if one were driving and controlling the vehicle.) • Now that Fred is in the driver’s seat, there is a lot less criticism about how things are being done. • Joan can’t wait to get into the driver’s seat and do what she can to turn things around.
McGrawhill's American Idioms And Phrasal Verbs