The 20-year-old took to Twitter to speak of his delight at his new deal, saying: "Really happy to sign a new three-and-a-half-year contract. I want to keep learning from the best and improving each day at the best club in the world."
Johnstone has been a regular for the Under-21s this season, since a finger injury cut short a planned three-month loan at Championship Yeovil after just one game in August.
The young shot-stopper has been with the Reds since 2009 and is awaiting his first senior appearance, though he was on the bench for two games in December: the Capital One Cup win over Stoke and the Barclays Premier League defeat of West Ham.
An FA Youth Cup winner with the Reds in 2011, Johnstone has also previously enjoyed loan spells with Walsall and Scunthorpe, and was England’s no.1 at last summer's Under-20 World Cup.