Maddison left the field in the 25th minute with an apparent leg injury after scoring the opening goal.
The 25-year-old was called up to the England squad earlier this week.
Maddison, who has now scored seven goals in the premier league this season, was able to walk off the pitch and didn't seem to be in any pain.
After Maddison's departure, West Ham were more involved in the game, but CraigDawson gave away a penalty which was well saved by Lukasz Fabianski.
Harvey Barnes finished off a lovely counter- attacking move when he scored the winning goal for the Foxes.
The Hammers have lost their last two games at London Stadium, including a last-minute goal against Crystal Palace last Saturday.
The two teams came into the game with four wins and two draws.
The Foxes have not conceded a goal in their last three away wins and got off to a great start by beating Newport.
Barnes drove forward and did well to keep the ball in play, his cut-back to Kiernan Dewsbury-Hall shifted along to Patson Daka, who miscued his kick and Maddison hammered it into the roof.
It was Maddison's 40th goal of the season and made him the second leading scorer in the league.
The visitors had a terrific effort from range when Daniel Amartey ambled through the middle of the pitch and hit a rocket from 20 yards out, forcing a big save from Fabianski before Tielemans missed the penalty.
The second half brought wholesale changes for Brendan Rodgers' side as they looked to see the game off and substitute Ayoze Perez set up Barnes to slide home.
After winning just one of their first 10 games this season, the Foxes have won four of their last five and are now in the top half of the table.
There's more to come.
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