Clare Cunningham secured a one-two finish for Great Britain with silver in the women's PT4 in Hyde Park.
"I was a bit disappointed with the swim, but the bike and run went well," Steadman said.
Melissa Read won GB's second gold of the event, edging out Joleen Hakker of the Netherlands in the women's PT5.
Steadman, 22, crossed the line in one hour nine minutes 26 seconds, almost five minutes ahead of Cunningham.
"I like the course and I had a lot of family here today and I could see them on the side, so it was quite nice," Steadman added.
Great Britain's Lauren Steadman continued her build-up to the Rio Paralympics as she reclaimed her London World Series Para-triathlon title.