Sir Attenborough's debut post said: "Saving our planet is now a communications challenge." Last October, Friends star Aniston reached the milestone in five hours and 16 minutes.
In his first video, the veteran broadcaster told followers: "I am making this move and exploring this new way of communication to me because, as we all know, the world is in trouble. Continents are on fire. Glaciers are melting. Coral reefs are dying. Fish are disappearing from our oceans. The list goes on and on. Saving our planet is now a communications challenge."
Sir David's total following rose to 2.5 million within 24 hours.
"Social media isn't David's usual habitat," wrote collaborators Jonnie Hughes, a BBC film-maker, and Colin Butfield, of the World Wildlife Fund. "So while he's recorded messages solely for Instagram, like the one in this post, we're helping to run this account."
Sir David's Instagram debut precedes the release of a book and a Netflix documentary, both titled A Life On Our Planet. The film will see him reflect on his career and the decline of the planet's environment and biodiversity, which he has observed first-hand.