While composing his biographical works Campbell remained active in the House of Lords and spoke frequently against legislation proposed by Sir Robert Peel's government. When the Whigs returned to power under Russell in 1846, Campbell became a member of the cabinet as Chancellor of the Duchy of Lancaster, and undertook some of the duties of the ailing Lord Chancellor, Lord Cottenham. On the resignation of Lord Denman in 1850, Campbell was appointed Chief Justice of the Queen's Bench. Although well versed in the common law, Campbell was criticised for attempting to influence juries in their estimate of the credibility of evidence, as was seen in the 1852 Achilli case. He assisted in the reforms of special pleading at Westminster, and had a recognised place with Brougham and Lyndhurst in legal discussions in the House of Lords.