Johnston has, of course, trained more winners than anyone else in Britain – and has had horses placed in the world's oldest Classic, including Bandari, favourite when third to the last northern-based winner Bollin Eric in 2002.
The Middleham handler has had many great stayers over the years. But success in Doncaster's Classic, which forms part of the Long Distance category of the Qipco British Champions series, has eluded him.
Nayef Road won the Group Three Qatar Gordon Stakes at Goodwood last month. But Johnston's principal hope on Saturday is Sir Ron Priestley, who was making it five wins from six starts this year when landing the Group Three Ladbrokes March Stakes at the Sussex venue on his latest start.
The Australia colt is owned by Paul Dean, one of Johnston's longest-standing patrons, and will be ridden by 49-year-old Franny Norton – whose decorated CV does not yet include a Group One winner.
Johnston said: "Sir Ron Priestley is going to have to up his game considerably – but he's done everything else we've asked of him, and so he's got to have a shot at the St Leger.
"We are under no illusions about the fact he will need a personal best to win.
"He won a Group Three race on his last outing, while this is a Group One with Classic horses in there and a much taller order. But he stays the trip well, and it's not impossible he could bridge the gap."
Victory would be especially sweet for Johnston, and all concerned.
He added: "The St Leger is a race I would dearly love to win – and it would be fantastic for Paul and Franny, who's as good as ever, to get a big one.
"Paul's a tremendous owner and has been with us almost as long as I've been training. I always say bookmakers make great owners, because they've made their living out of the fact that we don't know what is going to win."