On his return to Europe he was employed in various missions, and he published about this time "Naval Evolutions", a controversial work dealing with the question of breaking the line. From 1835 to 1840 Douglas, now a GCMG, was Lord High Commissioner of the Ionian Islands, where, amongst other reforms, he introduced a new code of laws. In 1837 he became a Lieutenant-General, in 1840 a KCB, in 1841 a civil GCB, and in 1851 a full general. He was given the colonelcy of the 99th (Lanarkshire) Regiment of Foot from 1841 to 1851 when he transferred as colonel to the 15th (the Yorkshire East Riding) Regiment of Foot, a position he held until his death in 1861.