During World War II, Darby was appointed honorary surgeon to King George VI.

Darby and his assistants (including the ship’s chaplain, Reverend Little) worked solidly for several hours in confined conditions below deck, attending to injured crew members.

They also attended to the German crew from the Emden who were brought into the Sydney’s sick bays, many severely injured.

Here is a list of some of Dr Darby’s medications and supplies on board the Sydney at the time of the battle:
Chloroform, 2 pounds
Drainage tubing, 1 yard
Absorbent wool, 18 pounds
Surgeon’s lint, 16 pounds
Boric lint, 10 pounds
Bandages 4 inch, 10 dozen
Iodine, 1 pint
Catgut ligature, 3 spools
Silkworm gut, 1 tube (50)
Horse hair, 1/8 hank
Hydrogen peroxide, 2 pounds
Acid Carbolic, 1 pound 

For his work as surgeon on the Sydney, Darby was awarded with promotion to staff surgeon, a jump of four years in seniority. It made him the youngest staff surgeon in the Navy.

Darby’s brothers fought in World War I, Private KH Darby in the Gallipoli campaign, and Captain HF Darby on the Western Front.

Darby was the first and youngest surgeon to join the Royal Australian Navy.