Sheikh Mohammed bin Zayed had been calling the shots for years from behind the scenes while his half-brother Sheikh Khalifa was sidelined by poor health.
Under his low-key direction, the United Arab Emirates has put a man in space, sent a probe to Mars and opened its first nuclear reactor, while using its oil-funded clout to develop a more assertive foreign policy.
Closely allied with Saudi Arabia, it has emerged as a leader of a reshaped Middle East since the retreat of traditional powers and the reduced involvement of the United States, forging ties with Israel and joining a war against Iran-backed militants in Yemen.
His election comes a day after the death of former leader Sheikh Khalifa bin Zayed Al-Nahyan.
Sheikh Khalifa, who was in office during a time of rocketing fortunes for the UAE but was rarely seen in public, was laid to rest in Abu Dhabi's Al Bateen Cemetery just hours after his death was announced, in accordance with Muslim tradition. His funeral prayers were conducted by his successor Mohamed bin Zayed.
The most visible testament to Sheikh Khalifa is the world's tallest building, Dubai's Burj Khalifa, which was renamed after he bailed out the debt-hit emirate when the global financial crisis struck in 2009.
As tributes poured in from abroad, US President Joe Biden called Sheikh Khalifa "a true partner and friend of the United States". Neighbouring Saudi Arabia suspended sports and entertainment events and Oman, Lebanon and Kuwait also ordered three days of mourning.
In a demonstration of the UAE's diverse allegiances, Russian President Vladimir Putin also paid tribute, as did Iran's foreign minister and Israeli Prime Minister Naftali Bennett.
Marking a day of national mourning, the Indian flag flew at half-mast at the Parliament in New Delhi.
