The 20-year-old has proved to be a revelation in his first full season, scoring 16 tries in 21 appearances, and that brought him to the attention of rival Super League clubs, including Wigan.
Johnstone, who came through Trinity’s academy, was already contracted to the end of 2018 but has now signed a new four-and-a-half-year deal.
Wakefield chairman Michael Carter said: “I am delighted to have agreed a new deal with Tom that keeps him at the club for the next four seasons. It might just put a few rumours to bed for a while.
“It is vitally important as a club that we manage to retain our academy products. Tom has had a fantastic year, is still learning and hopefully will become a much better player over the next four years.”
The news is a timely boost for Wakefield on the eve of their Cup semi-final against Warrington at Leigh on Saturday, although Johnstone will miss that game with a leg injury.
Johnstone said: “I am very happy to be staying at the club for another four years.
“They are a great set of lads to play with and we have got awesome fans who follow us in numbers week in week out. I am really looking forward to the next few years, especially working with Chezzy (coach Chris Chester).”