Jonny Brownlee finished third in the men’s event.
Learmonth, part of Britain’s gold medal-winning mixed relay team at the Olympics, used her prowess on the swim to comfortably hold off Tokyo individual silver medallist Georgia Taylor-Brown and Vicky Holland at West India Quay.
The British contingent received plenty of backing from a sizeable crowd, with Super League using an innovative ‘triple mix’ format that juggles the traditional swim-bike-run format in three short stints.
Taylor-Brown won the first swim-bike-run before Learmonth came out on top of a run-bike-swim that saw her friend and compatriot pay for a late crash on the bike.
The final leg was bike-swim-run, and Learmonth, who finished ninth in the individual race in Tokyo, was never close to being caught after pulling away from Holland in the water.
The men’s event was fast-paced and close, with overall honours going to Tokyo bronze medallist Hayden Wilde from New Zealand after he caught and then passed Brownlee and France’s Vincent Luis on the final run.
It was another strong showing from Brownlee, whose performance in the relay in Tokyo convinced him to delay his Olympic retirement for now.
Alex Yee the individual silver medallist in the Japanese capital, was the second British man across the line in fifth.