Early life
Born into poverty at Hoxton, London, the son of a solicitor's clerk, he worked in turn as an office errand-boy (from the age of 12), coal miner's clerk, and (at 17) a soldier with the Seventh Dragoon Guards stationed in Dublin (which was at that time part of the United Kingdom). He resigned from the army in 1853.

Activism and journalism
By this time a convinced freethinker, Bradlaugh returned to London in 1853, and became a pamphleteer and writer about "secularist" ideas under the pseudonym "Iconoclast". He gradually attained prominence in a number of liberal or radical political groups or societies, including the Reform League, Land Law Reformers, and Secularists. He was President of the London Secular Society from 1858. In 1860 he became editor of the secularist newspaper, the National Reformer, and in 1866 co-founded the National Secular Society, in which Annie Besant became his close associate.

In 1868, the Reformer was prosecuted by the British Government for blasphemy and sedition. Bradlaugh was eventually acquitted on all charges, but fierce controversy continued both in the courts and in the press. A decade later (1876), Bradlaugh and Besant decided to republish the American Charles Knowlton's pamphlet advocating birth control, The Fruits of Philosophy, or the Private Companion of Young Married People, whose previous British publisher had already been successfully prosecuted for obscenity. The two activists were both tried in 1877, and Charles Darwin refused to give evidence in their defence. They were sentenced to heavy fines and six months' imprisonment, but their conviction was overturned by the Court of Appeal on a legal technicality.

Politics
Bradlaugh was an advocate of trade unionism, republicanism, and women's suffrage, but unlike Besant, he opposed socialism. He was a supporter of Irish Home Rule, and backed France during the Franco-Prussian War. He took a strong interest in India.

Parliament
In 1880 Bradlaugh was elected Member of Parliament for Northampton, and claimed the right to affirm (instead of taking the religious Oath of Allegiance), but this was denied, and he subsequently offered to take the oath "as a matter of form". This offer, too, was rejected by the House. Because a Member must take the oath before being allowed to take their seat, he effectively forfeited his seat in Parliament. He attempted to take his seat regardless, was arrested and briefly imprisoned in the Clock Tower of the Houses of Parliament.

His seat fell vacant and a by-election was declared. Bradlaugh was re-elected by Northampton four times in succession as the dispute continued. Supporting Bradlaugh were William Gladstone, George Bernard Shaw, and John Stuart Mill, as well as hundreds of thousands of people who signed a public petition. Opposing his right to sit were the Conservative Party, the Archbishop of Canterbury, and other leading figures in the Church of England and Roman Catholic Church.

On at least one occasion, Bradlaugh was escorted from the House by police officers. In 1883 he took his seat and voted three times before being fined £1,500 for voting illegally. A bill allowing him to affirm was defeated in Parliament.

In 1886 Bradlaugh was finally allowed to take the oath, and did so at the risk of prosecution under the Parliamentary Oaths Act. Two years later, in 1888, he secured passage of a new Oaths Act, which enshrined into law the right of affirmation for members of both Houses, as well as extending and clarifying the law as it related to witnesses in civil and criminal trials (the Evidence Amendment Acts of 1869 and 1870 had proved unsatisfactory, though they had given relief to many who would otherwise have been disadvantaged). The later history of the affair and its constitutional importance in described in Bradlaugh v. Gossett.

Death
Bradlaugh's funeral was attended by 3,000 mourners, including Mohandas Gandhi. He is buried in Brookwood Cemetery.