In 1853, whilst on leave from his role with the Board of Trade, he was in Constantinople when Turkey declared war on Russia. The British Ambassador, Lord Stratford de Redcliffe, took up Simmons' offer of his services, requesting that he report on Turkish defences and their ability to resist a Russian advance. Promoted to captain on 17 February 1854, he became British Commissioner with the Turkish Army providing advice to General Omar Pasha as the Crimean War started. He assisted the Turks at the defence of Silistra in June 1854 and then led them at the Battle of Giurgevo in July 1854; having been promoted to brevet major on 12 July 1854 and to brevet lieutenant colonel on 14 July 1854, he landed with the Turks at the Battle of Eupatoria in February 1855 and remained with them for the Siege of Sevastopol in the Summer of 1855.