He served as a Junior Naval Lord under both Liberal and Conservative administrations and was put in charge of organising British and French transports during the Crimean War.

He became Commander-in-Chief, North America and West Indies Station and in this role he acted with diplomacy, especially in response to the Trent Affair on 8 November 1861 during the American Civil War, when USS San Jacinto, commanded by Union Captain Charles Wilkes, intercepted the British mail packet RMS Trent and removed, as contraband of war, two Confederate diplomats, James Mason and John Slidell.

[1] After initial training at the Royal Navy College at Portsmouth he joined his father's flagship, the fourth-rate HMS Leander, on the North American Station in 1819.

[5] Promoted to commander on 25 November 1830, he joined the sloop HMS Snake on the West Indies Station in December 1836 and was employed capturing slave-traders.

[6] Promoted to rear-admiral on 20 January 1858[7] and appointed Knight Commander of the Order of the Bath (Civil) on 20 December 1858,[8] Milne became Fourth Naval Lord in the second Derby–Disraeli ministry in April 1859.

[4] Milne became Commander-in-Chief, North America and West Indies Station, hoisting his flag in the second-rate HMS Nile, in January 1860: in this role he acted with diplomacy, especially in response to the Trent Affair on 8 November 1861 during the American Civil War, when USS San Jacinto, commanded by Union Captain Charles Wilkes, intercepted the British mail packet RMS Trent and removed, as contraband of war, two Confederate diplomats, James Mason and John Slidell.

The low freeboard, fully rigged turret-ship Captain had joined the Channel Squadron a short time before, and the combined fleet put to sea from Vigo".

[13] On 6 September "the fleet was sailing in two columns on the starboard tack in a fresh north-west breeze, and Sir Alexander Milne went on board the Captain in the afternoon to inspect her and see how she behaved at sea, as she was a novelty... During the time the Commander-in-Chief was on board the Captain the wind and sea had increased, and he had great difficulty in getting back to his own ship – the Lord Warden.