Nelson Mandela was a controversial figure who became an iconic one after spending more than a quarter of his life in jail. A strong thesis statement would acknowledge this controversy, opening an essay which seeks to understand and learn from Mandela's career rather than blandly admiring him. There are several ways to approach this. You might, for instance, look at the way in which Mandela was regarded as a terrorist for his use of violent methods of protest in the 1950s and 60s, but then became a more unifying figure as president. You might also examine the effect that more than twenty-five years spent in prison had on his character and his way of governing South Africa.
If you choose the first approach, the thesis statement might be something like this:
The career of Nelson Mandela shows that a radical can also be a great man.
This puts the Mandela controversy in stark terms, and should make for an interesting argument about how far violent methods are justified in the face of extraordinary oppression. For the second approach, you might argue something like this:
The Nelson Mandela who became President of South Africa in 1994 was not the same man who went to prison in 1964.
This would involve an examination of the change in Mandela's leadership style, from leading a movement to governing a nation.