Miley edged her London 2012 conqueror Yi Shiwen into silver as she romped home in a Championship Record time of 4:23.14minutes in Istanbul, Turkey.
The 23-year-old, who came home fifth in the Olympic 400m IM final back in the summer, qualified for the final fastest in the morning and kept up her form with a dominant swim, holding off Shiwen in the final stages while Hungary's Katinka Hosszu took bronze.
"The fact that I went faster than at Europeans is a massive bonus because that time was a really big surprise and I was worried I wouldn’t be able to repeat it here,” said Miley.
“I knew Ye Shiwen was going to come back on me in that last 100m so I was just hanging on for dear life.
“It’s a big confidence boost that no matter where we are in the programme, I can step up in the 400m IM and what we’re doing at the moment really seems to be working.”
And Miley was not the only medal on a successful first night for the Brits as Jemma Lowe took bronze in the 200m butterfly.
Lowe led at the 150m mark but faded in the closing lengths as Hosszu took gold and China's Liuyang Jiao silver.
“I am really happy,” said Lowe, who qualified seventh fastest for the final despite winning her heat.
“I was in a really slow heat in the morning, so I did not really know what to expect for this final.
“It made it difficult that I was in the outside lane, so I could not see how fast the others were swimming. But it is a final, so you always have to give your best shot.”
Also in action on Wednesday night were Georgia Davies and Lizzie Simmonds in the 100m backstroke semi-finals.
Davies lowered her own Welsh record to 57.41seconds, qualifying for the final third fastest, but it was less good news for Simmonds however whose time of 58.34seconds saw her just miss out on a spot in the final.
On the men's side Michael Jamieson took some solace from lowering his 100m breaststroke personal best to 58.56seconds, but that was only good enough for 12th overall and he too missed out on the final.
In the women's 4x200m freestyle relay, the British quartet of Rebecca Turner, Sophie Allen, Jazz Carlin and Miley had to settle for fifth in the last race of the night as the American team of Megan Romano, Chelsea Chenault, Shannon Vreeland and Allison Schmitt surged to gold ahead of Russia and China.
Elsewhere Ryan Lochte claimed double gold on the first night, romping to first in the 200m freestyle, edging out a resurgent Paul Biedermann in a time of 1:41.92minutes, before backing that up with a win as part of the American quartet in the 4x100m freestyle relay.
And the American was not finished there, also finding the energy to qualify for the 100m butterfly final with the fourth fastest swim of the night.
- Sports & Recreation
- Jemma Lowe