She served on the route until 1936, when a revised contract to carry the mails required a speed of at least 19 kn (35 km/h; 22 mph), which would result in a voyage to the Cape lasting no more than 13+1⁄2 days.

After undergoing sea trials on 26 June 1938, she returned to her original route on 8 July, setting a new record for the passage to the Cape of 12 days, 13 hours, 38 minutes.

Carnarvon Castle was at Cape Town at the outbreak of the Second World War, and was requisitioned by the Royal Navy on 8 September 1939.

Carnarvon Castle put into Montevideo for repairs with steel plate reportedly salvaged from the German cruiser Admiral Graf Spee.

With her trooping accommodation only marginally upgraded, she carried a flood of post war emigrants from Britain on low cost assisted passages to East and South Africa.