Johnstone, capped four times by England with his most recent in 2023, made 17 outings last season but lost his starting spot at relegated Wolves in December and has not featured since an FA Cup defeat to Liverpool in early March.
“I’m thrilled to be here and can’t wait to get started,” said the goalkeeper, who began at Manchester United and has also represented Aston Villa, West Bromwich Albion and Crystal Palace.
“I didn’t need anyone to talk me into it, but Joe Hart called the other day to underline what an incredible club this is, what a top dressing room they have, and how amazing the supporters are.
“You hear about the size of the club, but spending just a day here you feel it—the scale, the expectation. I relish that—pressure to win, pressure to perform—and we’re all aiming to lift trophies.”
Kasper Schmeichel had been Celtic’s first-choice keeper since summer 2024, but made his final appearance in February before retiring due to a shoulder injury.
Manager Martin O’Neill called Johnstone “a very good addition” and said Mheuka possesses “great quality”.
O’Neill added that new left-back Emenalo “has many attributes which I think can benefit us,” highlighting his speed on the ball, solid defensive work, and willingness to push forward into attacking areas.