Iheanacho put Leicester City in the lead just before halftime following a good work from Jamie Vardy.
Ayoze Perez increased Leicester City’s lead just after the hour-mark.
Vardy assisted Iheanacho for his second goal of the day.
The former Manchester City striker completed his hat-trick with 12 minutes to go receiving a pass from Wilfred Ndidi to finish a low drive pass Sheffield United goalkeeper.
Iheanacho has now scored in his last three Premier League matches against Burnley, Brighton and Sheffield United for the first time in his career.