During the Napoleonic Wars and afterward, Somerset served as the Duke of Wellington's military secretary. In 1852 he became master general of the ordnance and was created Baron Raglan. After Great Britain declared war on Russia (March 27, 1854), he led a force that was sent first to Turkey and then to the Crimea, where it landed (September 14) along with French and Turkish armies. The Allies won the Battle of the Alma River (September 20), but, forfeiting their advantage, they delayed their attack on Sevastopol until October and thus allowed the Russians to build up their defenses.

An ambiguous order by Raglan in the Battle of Balaklava (Balaklava, Battle of) (Oct. 25, 1854) led to the disastrous charge of the Light Cavalry Brigade under the 7th Earl of Cardigan. An inexperienced commander in chief in a difficult situation, Raglan was blamed (perhaps unjustly) for the campaign's lack of progress and for the suffering of his troops, who lacked adequate supplies and shelter during the winter of 1854–55. Gravely ill, he resumed the siege of Sevastopol in the spring but died shortly after a serious Allied defeat (June 18, 1855).

Raglan's name was applied to the raglan sleeve, which came into use in about 1855.