Mr McEwan, who was previously the bank's retail boss, succeeds Stephen Hester who led the bank for five turbulent years following the departure of his disgraced predecessor Fred Goodwin.
Mr McEwan will be paid £1 million a year plus £350,000 in lieu of a pension.
He takes the reins at a time when RBS is providing key endorsement for the Government's Help to Buy housing mortgage guarantee scheme, which is being brought forward despite concerns it may stoke a housing bubble.
He has been at the bank since August 2012 joining from Commonwealth Bank of Australia where he was group executive for retail banking services for five years.
Before that Mr McEwan, who is married with two children, was executive general manager in charge of its branch network, contact centres and third party mortgage brokers.
He worked in the insurance and investment industries both in Australia and New Zealand for more than 25 years.
He spent 18 years in senior executive roles including managing director of stockbroking business First NZ Capital Securities and chief executive of National Mutual Life Association of Australasia Ltd/AXA New Zealand Ltd.
His predecessor's exit was announced last June amid reports of a rift with Chancellor George Osborne.
Mr Hester had taken over at the bank in November 2008 in the midst of a calamitous period which saw it rescued by the Government, which still holds an 80% stake.
Its collapse followed the disastrous 2007 takeover of Dutch bank ABN Amro in a £49 billion deal that weakened its capital position and left it highly vulnerable to the looming credit crunch.
Mr Goodwin was widely blamed for the catastrophe and later stripped of his knighthood, and Mr Hester was parachuted in to put the bank back on an even keel.
Tens of thousands of job cuts followed while non-core assets worth more than £200 billion including a chain of 900 pubs and an aircraft leasing business were sold off.
RBS also began to sell off insurer Direct Line. Last week it announced a £600 million deal to hive off 314 bank branches under the revived Williams & Glyn's brand.
As Mr McEwan's appointment was announced in August, the bank said it had swung back out of the red with half-year pre-tax profits of £1.4 billion.
At the weekend he welcomed the announcement that the Help to Buy scheme was being brought forward, as RBS announced that it would introduce a range of 95% loan-to-value mortgages under the scheme.
Opening hours at its branches, which include the NatWest network, would be extended to cope with demand, with an aim to sign up 25,500 customers.
Mr McEwan's interests include water skiing, cycling, reading and spending time with his family.