The remains of the wealthy man aged 25-35 were found in a tomb close to the River Nile in Sudan by Durham University student Michaela Binder last year.
The bones showed evidence of metastatic carcinoma - cancer that has spread from where it started.
Analysis proved it came from a malignant soft-tissue tumour and spread across large parts of the body, making it the oldest "convincing" example of metastatic cancer ever found, said the authors of the study, which is published in the academic journal PLOS One.
Researchers from Durham University and the British Museum said the discovery would help to explore underlying causes of cancer in ancient times and provide insights into the evolution of cancer in the past.
Although it is one of the leading causes of death today, it is extremely rare in archaeological finds compared to other detectable killers.
This has led scientists to conclude cancer is a product of modern lifestyles and increased longevity.
But the discovery shows cancer did exist in the Nile Valley in 1200BC.
Ms Binder, the lead author from Durham University's Archaeology Department, said: "This may help us to understand the almost unknown history of the disease.
"We have very few examples pre the first millennium AD.
"We need to understand the history of the disease to better understand how it evolved and for that it is important to find more examples."