CAIRO — For more than a dozen years, Khairat el-Shater guided his family of 10 children, his sprawling business empire and Egypt’s largest Islamist movement, the Muslim Brotherhood, all from a prison cell.

Each week, he held court behind prison walls as young Muslim Brothers delivered to him dossiers about the organization that sometimes were as long as 200 pages. His corporate employees paid regular visits for strategic advice about his investments in software, textile, bus manufacturing and furniture companies and other enterprises. And before consenting to the marriages of his eight daughters, he met in prison with each of their suitors. Some of the grooms were prisoners with him, others made the pilgrimage, and five said their vows in his presence, behind bars.

Now Mr. Shater, 62, commands far wider influence.

One year after the overthrow of Hosni Mubarak brought Mr. Shater freedom, he has emerged as the most decisive voice in the leadership of the Brotherhood, the 83-year-old fountainhead of political Islam, at the moment when it has established itself as the dominant power in Egyptian politics.

With firm control of Egypt’s Parliament, the Brotherhood’s political arm is holding talks to form the next cabinet while Mr. Shater is grooming about 500 future officials to form a government-in-waiting. As the group’s chief policy architect, Mr. Shater is overseeing the blueprint for the new Egypt, negotiating with its current military rulers over their future role, shaping its relations with Israel and a domestic Christian minority, and devising the economic policies the Brotherhood hopes will revive Egypt’s moribund economy.