[4] While employed by the Cunard Line, Birnie was also a member of the Royal Naval Reserve, having been commissioned as a sub-lieutenant in 1904,[5] and promoted to lieutenant on 12 December 1907.
[6] During World War I, Birnie served in the Royal Navy, commanding HM Torpedo Boat 82 in 1915, and the destroyer Fairy in the North Sea in 1916–1917.
[7] On 18 November 1917, while in command of the patrol boat HMS P-57, Lieutenant-Commander Birnie sank a German submarine, UC-47, off Flamborough Head, Yorkshire, initially by ramming it at near full-speed, then dropping depth charges.
"[14] In February and March 1943, Birnie was in command of Convoy SC 121 from New York to Liverpool, sailing in the Norwegian merchant ship Bonneville.
[17] Birnie was an Elder Brother of Trinity House,[1] a non-departmental public body of the United Kingdom responsible for maritime safety.