Johnstone will succeed David McCullough, who leaves at the end of the month to become chief executive of the older people’s housing and care charity the Abbeyfield Society after more than five years at the RVS.
Johnstone, who left Samaritans in 2015, was until March interim chief executive of the Dover-based charity Migrant Help. She is chair of the Directory of Social Change and was last year appointed CBE for services to suicide prevention.
The charity, which employs 1,200 staff and has more than 25,000 volunteers, supports more than 100,000 older people each month.
Johnstone, who before joining Samaritans was interim chief executive of Capacitybuilders, the now defunct quango that provided infrastructure support to the voluntary sector, became a trustee of the RVS in January, a role she will relinquish before she becomes chief executive of the charity on 1 August.
Income at the RVS has been falling in recent years: it was £64.7m in the year to the end of March 2016, £10m less than in 2011/12.
Johnstone will be paid £136,500 for the role, the same amount that McCullough was on.
Richard Greenhalgh, chair of the RVS, said Johnstone’s record in building sustainable services, driving innovation and inspiring and enabling volunteers would be invaluable.
Johnstone said she was privileged to take up the role. "With an increasingly tough time for our society, I am looking forward to working with the staff and volunteers to ensure that Royal Voluntary Service can be the best it can be," she said.