Johnstone joined Manchester United at academy level; in the 2010–11 season, he was number one keeper in the academy, where he shared the duties the previous season. On 26 July 2011, Johnstone joined League One side Oldham Athletic on loan, as both of their first-choice goalkeepers were sidelined by injury. He made two appearances, both in pre-season friendlies; a 0–0 home draw with Burnley and a 1–0 away victory against Fleetwood Town, before returning to his parent club. On 9 September 2011, Johnstone joined League One side Scunthorpe United on a one-month loan after their first-choice keeper Josh Lillis suffered a long-term injury. The following day, he made his debut at the Glanford Park in a 1–1 draw against Sheffield United. On 19 October 2011, Johnstone dislocated his finger during a training session, forcing Scunthorpe United to search for an emergency goalkeeper once again. On 10 November 2011, Scunthorpe extended the loan until 9 December 2011, which was then extended to 10 January 2012. On 20 March 2013, Manchester United agreed for Johnstone to spend the rest of the season on loan to Walsall. Johnstone kept consecutive clean sheets in his first few games and was first-choice goalkeeper during his loan-spell. On 17 August 2013, Johnstone was loaned to Yeovil Town for a period of three months. He made his debut on the same day in a 2–0 loss to Burnley, but it was later confirmed that Johnstone had suffered a finger injury prior to his debut and returned to Manchester United for treatment.
2014–2017
On 31 January 2014, Johnstone was loaned to Doncaster Rovers for four weeks. With the first-choice keeper Ross Turnbull injured, Johnstone made his debut the following day at the Keepmoat against Middlesbrough. On 4 April 2014, Johnstone agreed a loan extension until the end of the 2013–14 season. On 27 October 2014, Johnstone again joined Doncaster Rovers on loan, initially until January 2015. On 12 January 2015, Johnstone joined his home-town club Preston North End on loan until the end of the 2014–15 season. On 25 May 2015, Johnstone kept a clean sheet in the League One play-off Final as Preston won promotion to the Championship with a 4–0 win over Swindon Town at Wembley Stadium. On Manchester United's pre-season tour of the United States in July 2015, Johnstone played as second-choice keeper ahead of Anders Lindegaard, covering for David de Gea while the Spaniard recovered from a knock. He started in the victories over Club América and San Jose Earthquakes and came on as a second-half substitute against Barcelona in a 3–1 victory. After rumours of De Gea's transfer saga to Real Madrid caused the Spaniard to be dropped from the Manchester United squad, Johnstone was named on the bench behind Sergio Romero for the first game of the season against Tottenham Hotspur. After De Gea returned to the United first-team, following the collapse of his move, Johnstone failed to make the bench again in 2015. On 31 December 2015, he re-signed for Preston on an initial 28-day loan, after Sunderland recalled Jordan Pickford on the same day. He made his first appearance of his second stint at Preston in the 2–1 win over Rotherham United at Deepdale. On 16 October 2016, Johnstone extended his contract at Manchester United until June 2018.
Loan to Aston Villa
On 5 January 2017, Johnstone joined Championship club Aston Villa on loan until the end of the 2016–17 season. He made his debut on 8 January 2017, in an FA Cup match against Tottenham Hotspur at White Hart Lane. During the period of this loan spell, he kept more clean sheets in the Championship than any other goalkeeper. In July 2017, he returned to Aston Villa on a loan deal until the end of the 2017–18 season.
West Bromwich Albion
On 3 July 2018, Johnstone signed a four-year contract with newly-relegated Championship side West Bromwich Albion, for a transfer fee reported to be in the region of £6.5 million. Upon joining the Baggies, he inherited the number 1 shirt, taking over from previous first-choice goalkeeper Ben Foster, who left to join Watford, after West Brom's relegation from the Premier League.
