Johnston will succeed violinist and founding member Oliver Heath, who is departing the ensemble.
Founded in 2002 at the Royal Northern College of Music, the quartet's other members include violinist Sara Wolstenholme, violist Gary Pomeroy, and cellist Christopher Murray.
Johnston will join the quartet’s debut at the Presteigne Festival and performances at Wigmore Hall, Palau de la Música Catalana, Festspiele Mecklenburg-Vorpommern, and Pierre Boulez Saal.
A recipient of the Gramophone Chamber Award for their recording of the complete string quartets of Sir Michael Tippett, the quartet became the first ensemble in 15 years to win the Royal Philharmonic Society’s Young Artists Award in 2013.
The ensemble’s next album will feature string quartets by Berg, Webern, and Schoenberg, with soprano Carolyn Sampson.
“It is an honour to become a member of the Heath Quartet, a group I have known and admired for so many years,” said Johnston.
“After twenty years of being involved in a four-way marriage, I have made the decision to see where different paths might lead me,” said Heath. “I am full of gratitude for all of the love the quartet has been shown on our epic journey together, and I look forward to supporting Sara, Marije, Gary and Chris in the next chapter of the quartet’s life.”