McCann was brought in to stem a seven-game losing run which cost Paul Hartley his job, and the change had an instant impact as Dundee climbed two places out of the bottom two in the table.
Dundee sat very deep for virtually all of the first half as McCann tried to stop the rot, but Motherwell could not take advantage of their possession and territory and were stunned on the stroke of half-time when Mark O’Hara netted from close range.
Marcus Haber scored a second after the break before helping O’Hara’s header into the net for the clincher as Motherwell again failed to deal with balls from out wide.
Louis Moult and Chris Cadden pulled goals back and Dundee got a stroke of luck when Tom Hateley cleared Ben Heneghan’s header from inside his goal when the score was 3-1.
McCann, who swapped the Sky Sports studio for the dugout, said: “I thoroughly enjoyed it. You do get caught up sometimes but it’s important you keep an element of control.
“Yes, there’s things that we need to work on. Motherwell probably bossed the game in terms of possession but I’m not going to be hard on the players. I know they can improve on that but that’s a natural element because we have been on such a bad run.
“But they were magnificent. It is three points but it’s a huge three points because it just changes momentum for us.”
Motherwell slipped into the relegation play-off place but there was no panic from manager Stephen Robinson, who bemoaned the officials’ failure to award Heneghan a goal.
“The performance itself I am happy with,” said Robinson. “I haven’t got loads of options on the bench, I haven’t got loads of strikers I can bring on and off.
“Moulty is still hobbling through it and he still produces a performance and a goal like that.
“But we have got it in our hands. We were well aware we wouldn’t go down this weekend and we wouldn’t stay up whether we won or lost and that hasn’t changed.”