It was a goal that deserved to have such a dramatic effect on the two sides' fortunes. The Norwegian, a Liverpool misfit brought back to London for a knock-down pounds 2.75m, profited from the willingness of Steffen Iversen to run wide and challenge for the ball, took his short pass and checked inside to launch an irresistible shot across Kevin Pressman and into the far corner. Leonhardsen contributed mightily on both flanks, but this was his defining moment.
"I think he's one hell of a buy," said his manager, George Graham. "He's a fantastic athlete. He'll come back all day, apart from the goals he contributes. He played a full match for Norway in midweek and I thought his contribution was first class."
Despite sitting on top of the tree, at least until this afternoon, with nine points from three wins out of their first four games, Graham was not, as you would expect, getting too carried away.
"The media gets far too excited. I'll look at the table at the end of October and see how we're going," he said. "We didn't play as well as we have in the last couple of games, but it's a good sign when you come away, don't play particularly well and win."
Spurs were not helped by the loss of John Scales, who strained a calf muscle in the warm up, forcing Graham to pitch Luke Young into the starting line up.
That enforced change saw Wednesday look the likelier in the early exchanges, but only until Chris Perry - another of Graham's intelligent acquisitions - took a grip on matters. If Gilles De Bilde's first touch had not let him down when put clear by Benito Carbone, Wednesday would have already been in the lead before Les Ferdinand soared to meet Mauricio Tarrico's long centre from the left and headed perfectly past Pressman.
It was a short-lived lead, De Bilde going down in Tim Sherwood's tackle as he drew back his foot to shoot five minutes later and Carbone putting away the penalty.
Darren Anderton shot just wide after the highly-effective Ferdinand had laid the ball off to him and Ferdinand thought he had an equally strong case for a penalty when he went over Pressman's outstretched arms.
But Leonhardsen's strike gave Spurs the half-time lead and, with Steffen Freund replacing the peripheral David Ginola at the interval, they usually looked capable of maintaining or even extending it.
Anderton, Freund and Perry all went close before a brief flurry from Wednesday during which their substitute, Andy Booth, had one header cleared off the line by Sherwood and saw another go narrowly wide.
Wednesday should have equalised with six minutes left when De Bilde laid the ball back for Ian Nolan. The full back, playing his first full game since breaking his leg against Spurs 18 months ago, stabbed it wide.
As if to show that Spurs generally had something in reserve, the hard- working Iversen chipped what would have been a spectacular third goal wide of a vacant net in the last minute.
"Looking at the League table can give you bad vibes," said Wednesday's manager Danny Wilson, whose supporters will be only marginally placated by the fact that Sheffield United are sitting at the bottom of the First Division.
He noted without surprise that Graham had taken off Ginola in order to shut the game down at half time. "But we have to have a belief that we can break them down and I don't think we showed that belief," Wilson said.