Ewan stayed tucked out of trouble during the majority of the race and only showed himself in the final 200 metres of the 51km circuit course.
"It's always hard going to races as a favourite, especially when there is such a top field here," Ewan said.
The Australian criterium champion finished ahead of Giacomo Nizzolo (Trek-Segafredo) and Adam Blythe (Tinkoff) in a time of 1hr 2min 25sec.
The win continues Ewan's outstanding start to the year and confirms the 21-year-old is the No.1 sprinter in the Tour field.
Already this month, Ewan has won the Mitchelton Bay Classic criterium series in Victoria and the national criterium championship.
— Tour Down Under (@tourdownunder) January 17, 2016
"I was probably more nervous today than I have been for any of the other races so far. We sat back for a lot of the race and just cruised around and they (Orica-GreenEDGE) were awesone. Once we needed to go to the front they kept me up there and led me out perfectly."
Orica-GreenEDGE rode superbly for Ewan, with Darly Impey and Michael Albasini prominent at the front of the peloton.
Newly-crowned Australian under 23 champion Chris Hamilton, riding for the composite UniSA national team, impressed with a solo breakaway.
But it was never going to stick as the sprinters' teams set up for the finish.
All eyes were on Ewan and IAM Cycling tried to upset Orica-GreenEDGE's plans with a big surge on the last of the 30 laps.
Despite the heavy traffic in the last few hundred metres, Impey put Ewan in a good position for the final blast to the finish.
While Simon Gerrans is the main rider for Orica-GreenEDGE at the Tour Down Under, they will try to set up Ewan for at least a couple of stage wins.