Muir Mackenzie was Clerk of the Crown in Chancery from 1885 to 1915 and served as Permanent Secretary to the Lord Chancellor from 1890 to 1915.

He was made a CB in 1893,[1] a KCB in 1898[2] and a GCB in 1911[3] and in 1915 he was raised to the peerage as Baron Muir Mackenzie, of Delvine in the County of Perth.

[4] In February 1924 Muir Mackenzie, then aged 78, was appointed a Lord-in-waiting (government whip in the House of Lords) by Prime Minister Ramsay MacDonald, head of the first ever Labour government, and was sworn of the Privy Council the same year.

They had one son and three daughters, one of them the violinist Dorothea Frances Muir Mackenzie(1881-1971), universally known as "Dolly", who studied with Eugène Ysaÿe and who in 1907 married the pianist Mark Hambourg.

Muir-Mackenzie died at his home in Cumberland Terrace, Regent's Park, London, in May 1930, aged 84, and was cremated at Golders Green Crematorium.