Moncreiff was Member of Parliament for Leith Burghs from 1851 to 1859, for Edinburgh from 1859 to 1868 and for Glasgow and Aberdeen Universities in 1868.
As lord advocate he was engaged as public prosecutor in important cases, notably the trials of Madeline Smith, Wielobycki, and the directors of the Western bank.
[3] In 1856, he defended the Scotsman in the libel action raised by Duncan McLaren, one of the members for the city of Edinburgh.
In January 1857, Moncreiff was presented with the freedom of his native city for the part he took in regard to the Municipal Extension Act.
[6] For 19 years Lord Moncreiff occupied the judicial bench, presiding over the trials in the justiciary court of Chantrelle (1878), the City of Glasgow Bank directors (1878), the dynamitards (1883), and the crofters (1886).
Moncreiff also published anonymously in 1871 a novel entitled A Visit to my Discontented Cousin, which was reprinted, with additions, from Fraser's Magazine.
On 23 May 1871 he was created a baronet of Kilduff in the County of Kinross in the Baronetage of the United Kingdom.
[7] On 9 January 1874 he was created Baron Moncreiff, of Tulliebole in the County of Kinross in the Peerage of the United Kingdom;[8] in 1878 he was appointed a royal commissioner under the Endowed Institutions (Scotland) Act, and in 1883 he succeeded his brother as 11th baronet of Moncreiff.
[10] In September 1888, Moncreiff resigned the position of Lord Justice Clerk, and took up the preparation of his Memorials.