Kelechi Iheanacho's fortuitous second-half strike broke the resistance of League Two Walsall and sent Leicester into the FA Cup fifth round.
After Youri Tielemans had missed a penalty on a testing afternoon for the Foxes, Iheanacho's effort from the edge of the box deflected off Brandon Comley and looped over Saddlers goalkeeper Owen Evans in the 68th minute.
It was the Nigeria international's 17th goal in 23 FA Cup appearances - the most by any player since Iheanacho made his debut in the competition in 2016.
Walsall, one of the form sides in League Two, had defended admirably against the 2021 Cup winners, cheered on by a capacity crowd at a sold-out Bescot Stadium.