Laura Mongan has become the first woman to train the winner of the St Leger with Harbour Law swooping late in a dramatic edition of the world's oldest Classic at Doncaster.

Odds-on favourite, the Aidan O'Brien-trained Melbourne carnival entry Idaho, lost his jockey Seamie Heffernan 600 metres out when he appeared to take a false step.

The race suddenly opened up, with champion jockey Silvestre de Sousa seizing the initiative on Ventura Storm but Harbour Law overhauled him to win by three-quarters of a length.

Heffernan appeared to escape serious injury but was taken to hospital as a precaution because of a bang on the head.

"It was brilliant! I think I screamed a lot. I'm in shock," Mongon said.

"It was a really professional performance by the horse, we knew he'd go somewhere in life.

"It proves we can do it with the ammunition. It's great to have a horse like this and bring him here for a race like this.

"To go down in history as the first woman to win it is amazing, at least I've done something right.

"We've felt all along that whatever he did today, he'd be even better next year."

Mongan is married to former jockey Ian, who won the Juddmonte International on Twice Over for the late Sir Henry Cecil.

"He's the easiest horse to train. Henry Cecil said to me good horses make good trainers, and he is by far the easiest horse we've had to train," he said.