He joins the charity as the Department of Health attempts to inform how health professionals, including GPs and Primary Care Trusts, should deal with the condition under its 'musculoskeletal framework'.
'We need to get people to understand the impact of arthritis,' said McDonald.
'It affects nine million people but there is a tendency for people to think of it as a minor and inevitable condition of getting old.'
McDonald replaces Neil Betteridge in the role, who was promoted to chief executive earlier this year.
McDonald was the MS Society's head of policy and campaigns for five and a half years, overseeing its campaign on access to beta interferon drugs.
He was previously at The Stroke Association.