Brenton Harrison Tarrant, 29, pleaded guilty to 51 counts of murder, 40 counts of attempted murder and one count of terrorism.
The killing spree was the deadliest in New Zealand’s modern history and prompted the government to rush through new laws banning most semi-automatic weapons.
Tarrant was scheduled to go to trial over the charges in June.
His change in plea came as a surprise and relief to survivors and relatives of the victims.
A sentencing date has yet to be set.
Tarrant faces life imprisonment on the charges.
The plea came at a hastily arranged court hearing at a time that New Zealand was beginning a four-week lockdown to try and combat coronavirus.
The lockdown meant that Tarrant appeared in the court from his jail cell via videolink and that only a few people were allowed inside the courtroom.