Key Taliban members decided to keep the death of their leader a secret for more than two years because the war in Afghanistan was entering a critical phase and most foreign troops were preparing to withdraw at the time, the group said on Monday.
News of Mullah Omar's death leaked last month amid confusion over where and when the one-eyed militant leader died, but on Monday the Taliban for the first time revealed April 23, 2013 as the date of his death.
The Taliban, who have not previously detailed the reason for the decision to keep Omar's death a secret, did not say who was party to the agreement.
"One of the main reasons was the fact that 2013 was considered the final year of power testing between the mujahidin and foreign invaders," the Taliban said in the document.
The Islamist insurgent group was ousted by a US-led military coalition in 2001, and is waging an increasingly violent war against Afghanistan's foreign-backed government.
"It was for these jihadi considerations that this depressing news was concealed in an extraordinary way up until July 30, 2015," the Taliban added.
Most Nato troops withdrew in 2014 and the remaining contingent is involved mostly in training efforts.
The US military has scaled back its involvement in combat to limited airstrikes and a separate counter-terror mission involving several thousand troops.
The statement on the website, controlled by Mansoor's supporters, played down the prospect of a serious rift within the Taliban leadership, saying religious scholars and hundreds of thousands of "ordinary people" backed his appointment.
It gave details of Mansour's life, from his birth in Kandahar in 1968 to his early studies, which were interrupted by the Soviet invasion in 1979.
The statement presented him as a capable military leader, twice wounded in combat, who helped rebuild the Afghan air force when the Taliban were in power.
It also described him as an attentive listener, who dresses neatly and avoids extravagance.