The son of an English sculptor and his Irish wife, Pearse became a director of the Gaelic League (founded 1893 for the preservation of the Irish language) and edited (1903–09) its weekly newspaper, An Claidheamh Soluis (“The Sword of Light”). To promote further the Irish language as a weapon against British domination, he published tales from old Irish manuscripts and a collection (1914) of his own poems in the modern Irish idiom. He founded St. Enda's College, near Dublin (1908), as a bilingual institution with its teaching based on Irish traditions and culture.

On the formation (November 1913) of the Irish Volunteers as a counterforce against the Ulster Volunteers (militant supporters of the Anglo-Irish union), Pearse became a member of their provisional committee, and he contributed poems and articles to their newspaper, The Irish Volunteer. In July 1914 he was made a member of the supreme council of the Irish Republican Brotherhood (IRB). After the Irish Volunteers split (September 1914), he became a leader of the more extreme Nationalist section, which opposed any support for Great Britain in World War I. He came to believe that the blood of martyrs would be required to liberate Ireland, and on that theme he delivered a famous oration at the burial (August 1915) of Jeremiah O'Donovan, known as O'Donovan Rossa, a veteran of Sinn Féin.

As an IRB supreme council member, Pearse helped to plan (January 1916) the Easter Rising. On Easter Monday he proclaimed the provisional government of the Irish Republic from the steps of Dublin General Post Office. On April 29, when the revolt was crushed, he surrendered to the British. After a court-martial, he was shot by a firing squad. More than any other man, Pearse was responsible for establishing the republican tradition in Ireland.

Pearse's Collected Works appeared in 1917–22 (3 vol.) and again in 1924 (5 vol.), and his Political Writings and Speeches in 1952.