Transferred from the  Olympic and had a role as one of the two Verandah and Palm Court First Class Stewards. He abandoned the ship with the keys to the verandah café still in his pocket.  

He is most well known for reporting to have spoken with Thomas Andrews (The Ship's designer) in the First Class Smoke Room. Walter Lord's A Night to Remember places this meeting at 2.10am but Stewart claimed to have escaped aboard lifeboat 15 which left at 1.40am, furthermore Andrews was seen on the Boat Deck at 2:00am. 

Steward John Hart knew Johnstone well and was aboard lifeboat 15 and claimed to never have seen him.