After more than 70 years of filmmaking, Attenborough's instantly recognisable voice is synonymous with the story of nature.
He is still at the vanguard of efforts to protect the environment and has produced some of his most influential work in recent years.
In Britain, Attenborough's centenary is being marked with a week of special broadcasts on the BBC, a live concert at the Royal Albert Hall, events at museums, nature walks and tree planting.
"I had rather thought that I would celebrate my 100th birthday quietly, but it seems that many of you have had other ideas," he said in an audio message released by the BBC on Friday.
"I've been completely overwhelmed by birthday greetings from preschool groups to care home residents and countless individuals and families of all ages."
He thanked all those who had sent messages and wished anyone planning an event to mark the milestone "a very happy day".
Counting Britain's royal family, former US president Barack Obama and pop star Billie Eilish among his admirers, Attenborough's charisma, humour and warmth, alongside the depth of his knowledge and flair for storytelling, have made him a broadcasting superstar.
"Your ability to communicate the beauty and vulnerability of our natural environment remains unequalled," was how the late Queen Elizabeth summed up his achievements in 2019.
While Attenborough has been named the country's most admired man and the greatest living British cultural icon, friends say he rolls his eyes when he is labelled a "national treasure".
"What he feels is that he's a public servant. He feels that he had the unique opportunity to be the voice for nature, to tell everybody about the wonders of nature," Mike Gunton, a television producer who has worked with Attenborough many times, told Reuters.
As climate change has accelerated and the threat to much of the world has become more urgent, Attenborough devoted much of his 90s to raising public awareness.
Attenborough's 2017 blockbuster Blue Planet 2, which highlighted the scourge of plastic in the ocean, achieved some of the highest viewing figures on British television before being sold to broadcasters around the world.
Born on May 8, 1926, Attenborough spent his childhood collecting fossils, insects and dried seahorses.
His BBC career took off in 1954 when he presented Zoo Quest, which involved him travelling the world and bringing animals back to London Zoo.
By the 1970s he had risen to be program controller at the broadcaster but decided he wanted to return to making nature documentaries.
Screened in 1979 when he was 52, Life on Earth made him a household name.
He wrote the entire 13-hour script and travelled the world for three years to tell the story of evolution from simple organisms to humans.
Dozens of documentaries followed, including Blue Planet, Frozen Planet and Dynasties.
As the decades passed, his sense of the need to act only increased.
"How could I look my grandchildren in the eye and say I knew what was happening to the world and did nothing?" Attenborough said.
