Nolan, 34, served as player-manager for Leyton Orient in the second half of last season, but departed the club in April 2016 and has been locked in discussions with Notts County in recent weeks.
The club's new owner Alan Hardy revealed at a press conference that Nolan has now taken charge of first-team affairs, with the former Newcastle United and West Ham United midfielder replacing John Sheridan.
"Kevin is an outstanding leader. We have 20 games left and we need immediate change on the pitch to ensure our EFL safety," Hardy told reporters.
Nolan takes charge of Notts County with the club down in 22nd spot in the League Two table.