The 21-year-old from Abergavenny won Great Britain's fourth gold of the first World Championships on the road to the Rio de Janeiro Olympics of 2016, succeeding Victoria Pendleton as women's sprint champion.
Appendicitis and an Achilles injury hampered James in Olympic year, meaning she was only a spectating reserve for the London Games.
Now illness and injury free, James travelled to Belarus in the form of her life and has proved her undoubted potential with three medals so far. A fourth is possible on the fifth and final day today, in the women's keirin.
James came from behind to win the best-of-three final 2-1 against Kristina Vogel of Germany.
"It's just not sinking in," said James, who won team sprint and 500 metres time-trial bronze medals in the opening two days.
"I wanted to get top eight in everything. But to be standing on that podium tonight, it's such a good feeling. It's been a tough few years, especially last year with illness and injuries.
"It was hard when the Olympics were on, but it just gave me more motivation seeing people winning – it makes me want to be up there winning as well."
Olympic champion Jason Kenny missed out on a medal in the corresponding men's event, losing his quarter-final to Sam Webster of New Zealand 2-0.
The 24-year-old from Bolton, who won the men's keirin on day three, qualified seventh, but was unable to advance to the day five semi-finals, finishing seventh overall. He said: "There was not a lot more I could have done, there was no firepower left in my legs."
At the halfway stage of the women's omnium, defending champion and Olympic gold medal winner Laura Trott lies third behind London 2012 silver medallist Sarah Hammer of the United States.
Trott was third in the opening discipline, the flying lap, 10th in the points race and claimed her seemingly routine victory in the elimination race.
In the men's event, Britain's Jon Dibben finished eighth overall as Aaron Gate of New Zealand won gold.