The 19-year-old has already made 12 appearances for the Paisley side this season, scoring his first Premiership goal with a sweet strike from the edge of the box in a 2-1 defeat against Aberdeen last month.
And, after losing former academy graduate Kyle Magennis, St Mirren boss Jim Goodwin is delighted to get the talented teenager tied down.
He said: "Ethan is only 19 but has had a consistent run in the team in his favoured position. "He is very mature in his playing style, he reads the game well, covers the ground and has a great range of passing.
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