In 1784 Smith was elected as one of the two Members of Parliament for Sudbury in Suffolk. He was active in his support for the Whigs while in opposition. In 1790 he lost his seat at Sudbury and in the following January was elected as one of the members for Camelford. In 1796 he was again returned for Sudbury, but in 1802 accepted the invitation of radicals to stand for Norwich, although he was defeated in the election of 1806, which was fought on a local issue. However, enough Whigs were elected to form the next government, led by Lord Grenville. Smith was returned again in 1807 and 1812 and became a popular and outspoken radical Member of Parliament for Norwich, which was known for being a gathering place for dissenters and radicals of all kinds.
Unitarianism
William Smith held strong dissenting Christian convictions – he was a Unitarian, and was thus prevented from attaining the Great Offices of State. He nevertheless played a leading role in most of the great contemporary parliamentary issues, including the Dissenters' demands for the repeal of the Test and Corporation Acts. Although the campaigners were unsuccessful in 1787, they tried again in 1789. When Charles Fox introduced a bill for the relief of Nontrinitarianism in May 1792, Smith supported the Unitarian Society, publicly declaring his commitment to the Unitarian cause. The same year he became one of the founding members of the Friends of the People Society. In 1813 Smith challenged the established church, and was responsible for championing the Doctrine of the Trinity Act 1813, known as 'Mr William Smith's Bill', which, for the first time, made it legal to practice Unitarianism. He was a member of the Essex Street Chapel.
Abolitionism
In June 1787, Smith was one of the first to campaign for the abolition of the slave trade, becoming a vocal advocate for the cause. In 1790 he supported William Wilberforce in the slave trade debate in April. While he had been out of parliament he had given his support to Abolitionism by writing a pamphlet entitled A Letter to William Wilberforce, in which he cogently and convincingly summarised the abolitionists' arguments for abolition. Once the trade had been halted, he turned his attention to freeing those who were already slaves. In 1823 with Zachary Macaulay he helped found the London Society for the Abolition of Slavery in our Colonies, thereby launching the next phase of the campaign to eradicate slavery.
French Revolution
In the beginning, at least, William Smith was sympathetic to the revolutionary movement in France. He visited Paris in 1790, where he attended the 14 July celebrations, and later recorded his reactions to the momentous events he witnessed. In April 1791 he publicly supported the aims and principles of the newly formed Unitarian Society, including support for the recently won liberty of the French. Smith was swiftly gaining a reputation as a radical, even a Jacobin. Because he had business contacts and friends in Paris, he was more than once asked to act as a go-between for the government. In 1792 he arranged several meetings between William Pitt and Maret, Napoleon's foreign minister, in an attempt to avoid war.
Later life
He was elected a Fellow of the Royal Society in 1806 as "a Gentleman well versed in various branches of Natural Knowledge". As an MP, Smith witnessed the assassination of prime minister Spencer Perceval. Smith finally saw through parliament the repeal of the Test Acts in 1828. He died on 31 May 1835 in London, at the age of 78.
Family
On 12 September 1781 Smith married Frances Coape, daughter of John and Hannah Coape, both Dissenters. They had five sons and five daughters. Of the sons:
