Several riders slid and fell on wet and cobbled corners on the 32.4-km course, with defending champion Chloe Dygert ending up being the biggest casualty and having to settle for bronze.
Britain's Anna Henderson took second place, one minute 31.59 seconds behind Brown's winning time of 39 minutes 38.24.
Dygert hit the deck on a corner in the Bois de Vincennes, which effectively ended her chances of retaining her title and cost her second place as she ended 0.87 seconds behind Henderson.
"I had a small feeling that maybe I could push for the podium," Henderson told the BBC.
"To be second behind somebody like Grace Brown, I can't believe it! I burst into tears when I found out!
"All the effort this year has been worth it.
"It has been a really rough year, so to achieve a goal like this is unbelievable."
