In a hard-fought encounter against the Foxes, Kevin Keen's younger troops made an impressive start and created several half chances before taking the lead in unorthodox style just before the break, when Collington chased a back-pass and closed down the goalkeeper, who smashed his clearance straight against the Hammers forward and watched in horror as the ball bounced into the net.
Leicester came out in determined mood for the second half and pushed forward in search of an equaliser, but Hammers defended superbly and held on for the three points.
Said a delighted Kevin Keen afterwards: "It is a very good result and overall I would say it is the most pleasing win of the season.
"Our goal was fortunate to say the least, but we deserved to be in the lead after an excellent first half performance, and the team defended magnificently in the second half.
"Leicester really came at us, but we were extremely strong and, although it is hard to pick out individuals after a superb team performance, I must give special mention to young Liam Norval, an under-15 central defender who was very impressive in his first game at this level.
"All the players performed today, though, and this kind of victory is excellent for team morale."
Team: Reed, Wright, Henry, Parrington, Norval, Cohen, Tucker, Bunce, Shaw, Collington (Allen 85), Lee. Subs unused: Yao, Singh, Welsh, Behcet.
Meanwhile, the under-19s only had themselves to blame after going down against Wimbledon in a game they should never have lost.
Tony Carr had rested several regulars after their recent FA Youth Cup and reserve team endeavours - including central defensive pairing Anton Ferdinand and Elliott Ward, but it was at the other end of the field that things went wrong.
After enjoying the majority of possession and virtually controlling the match, Hammers wasted three golden scoring opportunities in the second half and were punished for their profligacy when the Dons snatched a scrappy winner 15 minutes from time.
A long-range effort was spilled by keeper George Sealey and a Wimbledon forward followed up to stab the rebound home.
Said Tony Carr afterwards: "It's very disappointing to have lost a game that should have provided three points. Although we didn't dominate for 90 minutes, we were fairly comfortable and just needed a goal to set us on our way.
"We opened them up three times in the second half, but their keeper made one good save and we wasted the other two.
"We had a fairly young side out because many of the senior boys have been involved with the reserves in the last couple of weeks, but that isn't an excuse and the youngsters who have come in have done quite well.
"It is in attack that we have had problems, and with Freddy Eastwood leaving the club earlier in the week, I have been slightly short in that department. Still, this is the first real blip we have had this season, so it is nothing to panic about.
"Glen Johnson was playing in this team at the start of the season, and now he is in the first team, so that is the most important thing. It is all about pushing players forward and that is what we are ultimately judged on."
Team: Sealey, McClenahan, Tattam, Laws (Ferdinand 75), Blewitt, Lumsden, Noble (McMahon 78), Carrick, Mehmet, Morris (Pearson 78), Cole. Subs unused: Jackson, Khan.