Jamie Vardy opened the scoring for the hosts at the King Power Stadium in the 23rd minute and they doubled their lead courtesy of Johnny Evans' goal three minutes later.
In the 36th minute, Iheanacho compounded West Brom's woes after combining with Vardy to complete their first-half dominance and end their back-to-back defeats in the league.
The effort also took the Super Eagles striker's tally to 13 goals in all competitions in 2021, making him the second-highest scorer for a Premier League club after Tottenham Hotspur's Harry Kane with 15 goals so far since the start of the year.
Across all competitions, Iheanacho has scored 16 goals for the Foxes – nine in the Premier League with three and four goals in the Uefa Europa League and the FA Cup, respectively.