The 18-year-old from Leicestershire outsprinted Norway's Eline Brustad and Italy's Anna Stricker to repeat her victory in Copenhagen last year.
Only Britain's Beijing Olympic gold medallist Nicole Cooke, who won back-to-back world junior road titles in 2000 and 2001, had previously achieved that feat.
The field whittled down over the 80km course which consisted of five laps and included the Bemelerberg and Cauberg hills.
And European champion Garner was given a perfect lead-out by Elinor Barker, who won Tuesday’s time-trial, and produced a burst of speed to take the gold.
"I was nervous before the race, as always, but the course really suited me," Garner said
"We trained a lot in these surroundings to get ready for the hills. Especially the Cauberg, which I could take very well.
"This win is a team effort more than anything. The lead out Elinor gave me was fantastic."
1. Lucy Garner (GBR) 2:11:26
2. Eline Brustad (NOR) same time
3. Anna Stricker (ITA) "
4. Sophie Williamson (CAN) "
5. Jessy Druyts (BEL) "
6. Rasa Pocyte (LTU) "
7. Sheyla Gutierrez (SPN) "
8. Cecile Ludwig (DEN) "
9. Emily Roper (AUS) "
10. Alicja Ratajczak (POL) "