In 1834 he accepted the post of second in command to Colonel F. R. Chesney in the famous Euphrates Valley Expedition, and was placed in charge of the magnetic experiments.
He showed himself a loyal assistant to his chief during the next two years of arduous labour and travel, and it was chiefly owing to Chesney's advocacy of his services that Estcourt was promoted major on 21 October 1836, and lieutenant-colonel by brevet on 29 March 1839.
Although he had had no experience of actual warfare, he had recently completed the demarcation of the southern border of New Brunswick, a task that had seen him direct in a harsh environment over 500 men.
[3] He owed this important post to the support given to his application by his friend Lord Raglan, who believed that his polished and gentle manners concealed real strength of character.
As adjutant-general he performed his duties efficiently during the weary months of waiting and sickness at Gallipoli and at Varna, and also at the battles of Alma and Inkerman.
[5] Lord Raglan was afraid to attend the funeral, for fear of showing his grief; but the last visit he paid before his own death, was to Estcourt's tomb.