Martin Boyle netted the opener in the 59th minute and substitutes Christian Doidge and McGinn struck in the closing stages as Hibs moved eight points clear of their opponents and into third place above Aberdeen.
Motherwell had shot up three places on Thursday after being awarded 3-0 wins over Kilmarnock and St Mirren after their opponents' Covid-19 breaches led to postponed games in September.
Those decisions, following a Scottish Professional Football League tribunal, were criticised by Hibs head coach Jack Ross, who also argued that 3-0 was an unrealistic scoreline.
The double boost gave Motherwell the chance to move two points off Hibs and they almost took advantage.
Mugabi later hit the post before Doidge settled the outcome and McGinn scored in the dying seconds.
Hibs lined up with a similar 4-3-3 set-up to their hosts and both teams cancelled each other out in the opening stages.
Motherwell gradually began to look more threatening and Watt bundled the ball wide after meeting Callum Lang's cross.
Hallberg and Watt shot over from half-chances at either end before Ofir Marciano beat away Campbell's powerful swerving strike from 25 yards.
Motherwell had the ball in the net following a Liam Polworth free-kick just before the break but referee Andrew Dallas spotted an infringement as Mugabi volleyed home from close range following Mark O'Hara's knockdown.
Watt had a great chance on the counter-attack after Stephen O'Donnell sprinted from box to box and set up the striker, who blazed over from 15 yards.
Archer made a great close-range stop from Boyle 60 seconds later following Paul McGinn's low cross.
But the Hibs winger netted moments later after collecting Hallberg's deep cross in space and firing in off the far post.
Mugabi was so close to an equaliser five minutes later when he headed Polworth's corner off the inside of the post.
Hibs lost Jamie Murphy and Hallberg to injury but Boyle came close with a deflected effort before Doidge collected a pass from Joe Newell 18 yards out and passed the ball into the net in the 88th minute.
Stephen McGinn then swept home fellow sub Jamie Gullan's cutback five minutes into stoppage time.