Millwall were always struggling to contain Orient after having Tony Craig sent off early on and went down to their newly-promoted London rivals. Defender Craig was dismissed just four minutes in for a challenge on Orient midfielder Shane Tudor.
With half-time looming, striker Lee Steele broke the deadlock after latching onto Michael Simpson's ball.
In the 78th minute, Tudor came in from the right and Gary Alexander side-footed home Orient's second.
Leyton Orient: Garner, Tann, Lockwood, Fortune, Mackie, Tudor, Simpson, Easton, Keith, Alexander, Steele (Ibehre 72).
Subs Not Used: Barnard, Morris, Partridge, Corden.
Booked: Fortune, Lockwood.
Goals: Steele 45, Alexander 78.
Millwall: Pidgeley, Ross, Craig, Whitbread, Shaw, Elliott, McInnes (Hackett 63), Ardley, Williams (Morais 63), Byfield (Hubertz 20), Brighton.
Subs Not Used: Robinson, Day.
Sent Off: Craig (4).
Booked: Hubertz, Williams, McInnes.
Att: 6,142
Ref: L Probert (Gloucestershire).
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