In 1854 he went out in command of the Royal Fusiliers to Turkey and the Crimea, with a reputation as martinet. At the battle of Alma his regiment was on the right of the Light Division, and became engaged with the left wing of the Kazan Regiment, a deep column of fifteen hundred men. The Fusiliers held their own against this column when the rest of Sir William Codrington's brigade had fallen back, and at length forced it to give way. The regiment lost twelve officers and more than two hundred men. Yea received a letter of congratulation from Sir Edward Blakeney, who had led the regiment at Albuera, and was now its colonel.
At the battle of Inkerman the Fusiliers, as part of Codrington's brigade, were on the slope of Victoria ridge, acting on the right flank of the Russians, but not very heavily engaged. Yea was mentioned in despatches of 28 September and 11 November, and was made brevet-colonel on 28 November. During the hardships of the winter his care of his men was exemplary.
In the summer he had command of a brigade of the Light Division during the Battle of the Great Redan. In the assault of the Redan on 18 June 1855, he led the column directed against the left face. It consisted of a covering party of a hundred riflemen, a ladder party of about two hundred, a storming party of four hundred men of the 34th, and a reserve of eight hundred men of the 7th and 33rd. Leaving the latter under cover for the time, he went forward with the rest. They had a quarter of a mile of open ground to cross under grapeshot. Yea reached the abattis with the wreck of his parties, but there he was shot dead. His body was brought in next day, and he was buried on the 20th.
Yea was praised by Lord Raglan, in his despatch of 19 June; and by Sir William Codrington, then commanding the Light Division, to Yea's sisters. His eldest sister put up a marble monument to him in his parish church of Taunton St. James's, Somerset. A headstone marked his grave in the cemetery at Sevastopol.
Yea was unmarried. His father survived him, dying on 20 May 1862, when the baronetcy passed to Lacy's younger brother, Sir Henry Lacy Yea (d. 1864), third and last baronet.
The township of Yea, Victoria, Australia, was named in honour of Colonel Yea