Brett rapidly made his mark in the House, and in early 1868, he was knighted and appointed Solicitor General under Benjamin Disraeli. On behalf of the Crown, he prosecuted the Fenians charged with having caused the Clerkenwell Outrage. In Parliament, he took a leading part in the promotion of bills connected with the administration of law and justice. In August 1868, he was appointed a Justice of the Court of Common Pleas. Some of his sentences in this capacity excited much criticism, notably so in the case of the gas stokers strike, when he sentenced the defendants to imprisonment for twelve months, with hard labour, which was afterwards reduced by the Home Secretary to four months.