in the driver's seat

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.

The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.