Just minutes into prosecutor Brad King's opening statement to jurors, Rod Ferrell's lawyer, William Lackay, said his client wanted to change his plea.
Mr Ferrell, who could face the death penalty, also pleaded guilty to armed burglary and robbery charges.
The teenager had pleaded innocent to two counts of first-degree murder for bludgeoning to death Richard Wendorf and Naoma Ruth Queen in November 1996, in their home in Eustis, Florida.
Mr Ferrell purportedly cut himself and sucked his cult members' blood. Court papers said that cult members would take drugs and have group sex.
- AP, FloridaReuse content