O'Connor, who claimed royal descent from the ancient kings of Ireland, practiced law but exchanged law for politics when he entered the British Parliament in 1832 as a member for County Cork. Unseated in 1835, O'Connor turned to radical agitation in England, although he continued to press Irish grievances and to seek Irish support. As a result of his humour, invective, and energy, O'Connor became the best known Chartist leader and the movement's most popular speaker. His journal, the Northern Star (founded in 1837), gained a wide circulation.

O'Connor's methods and views alienated other Chartist leaders, particularly William Lovett, but in 1841, after spending a year in prison for seditious libel, O'Connor acquired undisputed leadership of the Chartists. Failing to lead the movement to victory and vacillating in his attitude toward the middle class and toward the Charter (a six-point bill drafted and published in May 1838), O'Connor began to lose power, although he was elected to Parliament for Nottingham (1847). The failure of the Charter in 1848 marked the beginning of the end for O'Connor, whose egocentricity was already bordering on madness. Declared insane in 1852, he died three years later.