With 12 minutes gone, John Gilbert’s corner was met by Lexus Beeden and his header was cleared off the line.
Six minutes later, Maidstone took the lead. Jephte Tanga’s neat low shot from close to the edge of the box went into the right side of the net.
And it wasn’t long before the Stones added a second. Gilbert won the ball in midfield and exchanged a one-two with a trialist before dispatching a decent finish past the Millwall keeper.
Deon Moore then went close with a rising drive before Maidstone scored again on 54 minutes. Tanga found Gilbert and his low effort was accidentally scooped into the net by the keeper.
Millwall’s keeper did well to push a shot from Moore over the bar and from the resultant corner, he made another good save to deny David Agbontohoma.
Twelve minutes from time, Maidstone rounded off the scoring. The substitute trialist, who was wearing the number five shirt, finished from close range, after seeing his first effort blocked.
Maidstone United: Jephte Tanga 18, John Gilbert 20, 54, Trialist 78
Millwall U21: –
Maidstone United first half starting 11: Trialist, 2. David Sesay, 3. Leo Hamblin, 4. Lexus Beeden, 6. TJ Bramble, 7. Jephte Tanga, 10. John Gilbert, 15. David Agbontohoma, 17. Riley Court, 20. Deon Moore, Trialist.