Tony Carr's team gave a thoroughly professional performance to overcome a previously unbeaten Norwich side who have already beaten Liverpool in the FA Premier Youth Academy league this season.
The Hammers opened the scoring after 25 minutes, when McMahon collected the ball on the edge of the penalty area and crashed home a left-foot shot for his third goal in two games following last week's double strike against Watford.
Soon after, the lead was doubled when Mitchell Cole's dangerous cross was only parried out by the Canaries keeper and Mehmet was on hand to tap home.
Norwich stepped up the pace in the second half as they searched for a lifeline, but the Hammers defence held firm and ensured the excellent record continued.
Said Tony Carr afterwards: "It was an excellent performance against a very good Norwich side and the players are high in confidence at the moment.
"It has been an excellent start, we have scored 10 goals and conceded only one in four games.
"We are keeping clean sheets, scoring goals from midfield and generally playing very well - long may it continue."
Team: Jackson, Johnson, Tattam, Ferdinand, Ward, Lumsden, Allen, McMahon, Cole, Mehmet (Akinsete 75), Eastwood (Pearson 65). Subs unused: Fletcher, Morris, McClenahan.
Meanwhile, Kevin Keen's under-17s secured a valuable point against Norwich's East Anglian neighbours Ipswich thanks to a first goal of the season from Danny Bunce.
Hammers had started the better of the two sides, but it was the hosts who took the lead against the run of play when they headed home a corner in the 25th minute.
Five minutes before half-time, though, the visitors deservedly equalised when central midfielder Bunce fired in a low shot from the edge of the penalty area.
Both sides went close to snatching a winner in the second half but had to settle for a draw that, in the end, was probably a fair result.
Said Kevin Keen: "We played very well in the first half and were slightly unfortunate to find ourselves a goal down midway through the first half.
"As they did last week, though, the players showed great character to come from behind and get a result.
"It could have gone either way in the second half, both sides had chances, but we looked solid enough at the back and I was pleased with all the players."
Team: Reed, Parrington, Laws, Blewitt, Wright (Singh 80), Cohen, Bunce, Tucker, Noble, Lee (Stokes 75), Collington (Yao 75). Subs unused: Wayman, Ketchell.