Posh had to be patient after both men missed early openings against an Orient side clearly trying to respond to boss Martin Ling's criticism following a midweek Carling Cup disaster at Ipswich.
Orient forced Posh onto the defensive with Sean Thornton pulling the strings in midfield, well supported on the left by youngster Jason Demetriou, but apart from a succession of corners they never seriously troubled towering home goalkeeper Joe Lewis.
Darren Ferguson's men finally got their noses in front a minute before the break and it was all down to Mackail-Smith, who dispossessed a hesitant Brian Saah on the halfway line and cut-in from the right to beat Glenn Morris with a superb curling shot from the edge of the area.
Posh added a second four minutes into the second period with McLean opening his account for the campaign, taking two touches to control a free-kick from Dean Keates before turning to fire past a helpless Morris.
McLean should have had his second on 62 minutes only to stand on the ball when Mackail-Smith squared a short pass across the Orient goalmouth. Then Keates lifted a 25-yard free kick just over the crossbar.
Mackail-Smith's persistence earned him a second on 68 minutes when the little striker battled to win the ball, after a shot from Sergio Raul Torres was blocked and somehow managed to steer it into the net.
Morris denied George Boyd and McLean and Keates also went close as Posh continued to search for goals.