How many years was Nelson Mandela imprisoned?

How to get close to this number

Anchor on the release date and the apartheid span. He was released in 1990, four years before the system ended, and this site's companion question dates that system from 1948. An imprisonment covering most of the system's duration puts the answer in the high twenties.

The sequence in the explanation is what makes the figure historically remarkable: twenty-seven years in prison, then election as head of state four years after release. The gap between those two conditions is four years, which is less time than a single term of office — and it is the most compressed political transformation of the century.

The location matters to how the sentence is remembered: most of it on an island prison off a major city, visible from the mainland. A political prisoner held in sight of the country he would later lead is the kind of detail that makes a duration meaningful rather than merely long.