The 18-year-old Tasmanian claimed her third world championship title of 2014 after easily beating Denmark's Pernille Mathiesen.
In a sensational result, Australia finished with three riders in the top four after New South Wales's Anna-Leeza Hull collected bronze just half a second ahead of South Australia's Alex Manly who placed fourth.
"I just can't believe it. I don't even know how I am feeling right now," said Stewart, a dual representative for Australia in 2014 who added to the two gold - omnium and team pursuit - she won at the Junior Track World Championship held in Korea in August.
"Very overwhelmed, blown away, it's all a bit of a whirl wind right now to be honest.
"To win a world title, it is something you dream of. Didn't think I was capable of one, so to achieve a third in one year, it just shows hard work pays off."
Stewart was the first of three Australians, and the 25th of 49 riders, to set out on the 13.9km course on the wet Ponferrada roads. She was the fastest at the halfway mark before riding into the hot seat with a time of 20mins 08.39secs, the quickest by 26 seconds.
"I would have preferred to not be first rider out as I wanted to know the other rider's splits, but knew I just had to give it absolutely everything and not worry about anyone else," said Stewart, the reigning road national champion.
"The conditions weren't ideal for racing, but when I saw it was wet I was happy as I am confident in the rain. I tend to take it hard, I am more game in the wet and that's what I did today. I gave it everything."
Hull (NSW) found herself in fifth overall at the time check, 13 seconds slower than Stewart, however a strong second half saw her move provisionally in to the silver medal position behind her team mate with 20mins 21.70sec.
Pernille Mathiesen (DEN) moved in to the silver medal position, moving Hull down to the bronze medal spot, while 2013 bronze medallist Manly (SA) finished in a storm over the final few kilometres to post a time of 20mins 22.20secs, just off the podium by the narrowest of margins.
"I was an emotional wreck while I was in the hot seat as Mathiesen was coming to the line. I have never shaken as much in my life as I did then, I felt sick, my heart was in my mouth," revealed Stewart.
"Then when I saw her time, and she hadn't beaten mine, I knew only Alex was left out there and right then, I wouldn't have cared if Alex had come through for the win. That would have been fantastic too.
"We have all worked so hard and I am so proud of the girls and team. Thank you so much to our coach Donna (Rae-Szalinski) who has put in so much hard work."
Newcomer Hull, 17, was ecstatic after finding the podium while on debut.
"So happy, no idea what to expect come here for my first Championships, I can't believe it," said Hull, the reigning time trial national champion. "I was slightly nervous this morning, but when I got on the rollers and it all came together.
"To have all three of us in the top four is just amazing. We were hoping for three on the podium, it was so close for Alex, but she rode a great race so three in top four is a great result."
The trio will now switch their focus to the under 19 road race to be held on Friday 26 September.
Junior Women's Time Trial: 13.9km, Ponferrada
1 Macey Stewart (AUS) 20min 08.39sec
2 Pernille Mathiesen (DEN) 0:00:10
3 Anna-Leeza Hull (AUS) 0:00:13
4 Alexandra Manly (AUS) 0:00:13
5 Emma White (USA) 0:00:26
6 Greta Richioud (FRA) 0:00:26
7 Melissa Lowther (GBR) 0:00:27
8 Aafke Soet (NED) 0:00:28
9 Daria Pikulik (POL) 0:00:38
10 Daria Egorova (RUS) 0:00:44
The first five days of the UCI Road World Championships will be live-streamed on the Cycling Central website, while the final two days will be broadcast on SBS ONE and streamed online.