The animals began migrating from Kenya to Tanzania in September and were not expected to return until April.
Nicholas Murero, co-ordinator for the Mara-Serengeti ecosystem, said drought in Tanzania was the likely cause.
He told the BBC's Focus on Africa programme that he and his colleagues had "never experienced this before".
He said a lack of pasture in Tanzania due to the drought was probably the reason why they had made the journey north so early.
"They look very thin," he said.
The unusual occurrence is likely to have a positive effect on tourism in Kenya, less so for Tanzania, he notes.
The annual migration of more than one million wildebeest
between the Seregenti National Park in Tanzania and Kenya's Masai Mara
Game Reserve is a spectacle that draws tourists from around the world.
The sight of wildebeest crossing the crocodile-infested Mara River has been described as the seventh wonder of the world.