Dr Marshall is standing down after leading the committee since 2006. Dr McDevitt has been joint deputy chairman since 2009.
Dr McDevitt graduated from Glasgow medical school in 1985 and is active in Glasgow LMC.
He said: 'It is a privilege to be elected to lead Scottish GPC at such a challenging time for general practice. With falling budgets and rising demand it is a difficult financial climate for all parts of the NHS. However, I want to build on the strengths of Scottish general practice and to work with the government to ensure that GPs have a real say in the development of healthcare policy, both locally and nationally.
'General practice is at the heart of every town, village and city in Scotland and the strong links we have with our local communities and the relationships we build with our patients means that we must be at the centre of any primary care strategy.'
Dr McDevitt takes up his role on 23 August. Dr Marshall will continue to serve on GPC and as a member of the BMA's Scottish and UK Councils.