King was Member of Parliament (MP) for East Surrey from 1847 to 1874. He won some fame as an advocate of reform, being responsible for the passing of the Real Estate Charges Act 1854, and for the repeal of a large number of obsolete laws. Increasingly as politics in the United Kingdom turned toward left-right politics he sided with the mainstream progressive wing of the Liberal Party.
In 1837 he unsuccessfully contested East Surrey. He served as High Sheriff of Surrey in 1840. In the election of 1847 he ran again and this time was elected MP for East Surrey on 11 August. He retained his seat until more entrenched partisanship set in and a Conservative reaction defeated him at the general election in February 1874. He supported an alteration in the law of primogeniture for many sessions. On 15 March 1855 he delivered a speech in which he showed emphatically "the crying injustice of the law".
On 11 August 1854 he passed the Real Estate Charges Act, according to which mortgaged estates descend with and bear their own burdens. Formerly mortgagees in possession could seek an order in the court to trace certain contemporaneous held property in the hands of other relatives. In the session of 1856 he was successful in obtaining the repeal of 120 sleeping statutes which were liable to be put in force from time to time. He also waged war against the statute law commission, and more than once denounced it as a job. King introduced a bill for abolishing the property qualification of members, which passed the House of Lords on 28 June 1858, and in eight successive sessions he brought forward the county franchise bill, on one occasion, 20 February 1851, defeating and causing the resignation of the ministry led by fellow Whig, (Earl) Russell.
He succeeded in carrying through the House of Commons a bill for extending the £10 franchise to the county constituencies, so as to include every adult male who came within the conditions of the borough suffrage. He was also well known for his advocacy of the ballot and of the abolition of church rates, and for his strenuous opposition to the principle and practice alike of endowments for religious purposes. He died aged 74 at Brooklands, Weybridge, on 12 November 1885.
On 22 March 1836 King married Louisa Elizabeth, daughter of William Henry Hoare of Mitcham Grove, Surrey. She died in 1884. They had two sons and four daughters including: Hugh F. Locke King, entrepreneur who inherited the estate and used it to found and finance the creation of the Brooklands motor racing circuit and aviation field.