Slough went into the match without the services of club captain Gary MacDonald and Stuart Lake at centre-back, and also strikers Lewis Putman and Scott Harris, so were already up against it before Smith was forced off with a knee injury after half an hour. Partnering Smith in attack was Mpi, making his second appearance after re-joining the club. Guy Hollis and Billy Jeffreys returned to form yet another different centre-back partnership.
Over the last 15 years, the Rebels have had a miserable record against Chesham, and Matt Taylor went close to giving the Buckinghamshire side the lead on three minutes, with an improvised effort which Scott did well to turn away.
Scott also saved with his knees from Sam Youngs, but Smith had the clearest chance on 12 minutes as he was one-on-one with George Legg, but the keeper spread himself well to block the shot.
Chesham were more of a goal threat after this, and Mark Scott's goal led a charmed life. He dived all the way across his goal to save a placed free kick from Youngs, and the hosts whipped numerous balls across the face of goal without having the finishing touch on any of them.
After Smith left the field, Toby Little put in a low cross-shot which Scott turned away, and the Slough stopper then kept out a Ryan Blake effort with his legs.
Scott prevented a ragged Slough, who kept giving the ball away too easily, from falling behind again on 40 minutes when he superbly tipped over Blake's curling effort.
After a half-time dressing-down, Slough came out with much more purpose and almost took the lead through Billy Jeffreys, but his header was cleared off the line and into Legg's arms.
Legg needed to be alert on 55 minutes to tackle Mpi on the edge of the box as the striker's pace threatened the hosts. The ball ran on to Jamie McClurg, who found Warren Harris in space, but the winger dragged the ball wide.
Chesham's opening goal was against the general run of play when it arrived on 62 minutes, but was a fine effort from Pearce. He turned past his marker too easily and fired into Scott's bottom corner, leaving joint manager Jon Underwood to bemoan 'weak' defending after the match.
Nine minutes later, Scott proved his worth again with a fine save to keep the score at 1-0, whilst Slough could only muster a Harris shot which was deflected into the side netting.
Slough looked certain to be heading for another defeat before Mpi's heroics six minutes from the end of normal time. Ryan Parsons floated a cross into the box with his less favoured right foot, and Mpi guided the ball superbly into the corner of the goal with his head to ensure Slough went home with a point.
There was to be one further blow for Slough, though, as Scott collided with Chesham striker Mark Wadkins late on, leaving Scott having to be stretchered off and taken to hospital with a potentially serious knee injury.
Chesham United Line up
George Legg, Ashley Smith, Toby Little (c), Laurie Stewart, Nick Beasant, Matt Taylor, Dave Pearce, Sam Youngs, Ryan Blake, Brad Wadkins, Bruce Wilson. Subs: Curtis Ujah, Tommy Fletcher, Tashan Dwyer, Stephan Hamilton-Forbes, Ash Fongho.