Nelson Mandela remained in prison for treason for about 27 years.

Explanation:

Nelson Mandela, a prominent anti-apartheid activist and leader of the African National Congress (ANC), was arrested in 1962 and charged with treason. The treason trial began in 1963 and lasted until 1964. Mandela and his co-accused were accused of planning to overthrow the apartheid government through violent means.

During the trial, Mandela and his fellow defendants were found guilty and sentenced to life imprisonment. Mandela was sent to Robben Island prison, where he spent the majority of his sentence. Robben Island was a maximum-security prison located off the coast of Cape Town, South Africa.

Mandela's imprisonment on Robben Island was extremely challenging, both physically and emotionally. Prison conditions were harsh, and political prisoners, including Mandela, were subjected to hard labor and limited privileges. Despite these difficult circumstances, Mandela continued to be a symbol of resistance and leadership, even from behind bars.

During his time in prison, Mandela became a symbol of the anti-apartheid movement both within South Africa and internationally. His imprisonment garnered significant attention and support, with many activists and world leaders calling for his release.

After serving 27 years in prison, Nelson Mandela was finally released on February 11, 1990. His release marked a significant turning point in South African history and paved the way for negotiations between the apartheid government and the ANC.

Mandela went on to play a crucial role in the negotiations that led to the end of apartheid and the establishment of a democratic South Africa. In 1994, he became the country's first black president, serving as a unifying figure and promoting reconciliation and equality.

Nelson Mandela's long imprisonment for treason is a testament to his unwavering commitment to justice and equality. His resilience and determination during those years made him a global icon and a symbol of hope for oppressed people around the world.