Rudman, the defending World Cup champion and Olympic silver medallist from Turin 2006, saw her advantage cut in the final two runs but it mattered little as she beat Noelle Pikus-Pace of the USA by 0.57 seconds.
Another Brit, Lizzy Yarnold, just missed out on bronze as she was pipped to the final spot on the podium by Canada's Sarah Reid by just 0.13 seconds.
Rudman had claimed three wins and a second on her last four visits to St Moritz but she admitted she didn't expect to be so dominant at the World Championships.
"No I didn't at all. I train to train and I try and race to race and it went well yesterday [Thursday]," said Rudman. "I was a little bit more relaxed but I still didn't expect to win until it was over."
Rudman had held a one-second lead over Pikus-Pace after the first two runs in Switzerland having been fastest over both runs at the halfway stage.
And while Pikus Pace timmed Rudman's lead in the final two runs, the Briton's early advantage had given her a big enough cushion to hold on comfortably for gold.
She becomes the fourth British female to medal at the World Championships but the first to claim gold. Her male counterpart Kristan Bromley is the only other Briton to have won World Championship gold, a feat he pulled off back in 2008.
Yarnold, who won bronze last year, was third after run three but just missed out while the third Brit in the field in St Moritz, Donna Creighton, was 21st.
Alexander Tretyakov of Russia leads at the halfway stage of the men's event on home ice.
The bronze medallist from the 2010 Winter Olympics in Vancouver is nine hundredths ahead of Latvian Martins Dukurs, a winner of six of the seven World Cup events this season.
Sergey Chudinov, only 12th in the World Cup standings, also used local conditions to his advantage and is third, 0.65 behind the leader, and .61 seconds ahead of Martins' brother Tomass
Britain's 2008 world champion Kristan Bromley is back in eighth, 1.72 seconds off the pace, ahead of Saturday's final two runs.
1. Shelley Rudman (GBR) 4:38.60 (1:09.54 + 1:09.49 + 1:09.63 + 1:09.94)
2. Noelle Pikus-Pace (USA) +0.57
3. Sarah Reid (CAN) +1.41
4. Elizabeth Yarnold (GBR) +1.54
5. Melissa Hollingsworth (CAN) +1.75
- Sports & Recreation
- Sarah Reid
- Shelley Rudman