Steward, who is currently head of enforcement at the Hong Kong Securities and Futures Commission, will take over from activing executive Georgina Phillippou.
Barbara Frohn, who is currentyly managing director of Banco Santander, will be director of risk and compliance oversight for the regulator.
Both are expected take up their new roles in London in early autumn.
Martin Wheatley, chief executive of the Financial Conduct Authority, said: “These are two vital roles within the UK regulatory system and it says a lot about the FCA that we have been able to attract such high calibre candidates to fill them.
“Mark and Barbara have built up excellent reputations for their work in financial services across the globe, and I am delighted they have chosen to bring their experience to the FCA.”
The FCA has also made three director appointments in Marc Teasdale as director of market oversight within the enforcement and market oversight division, Jo Hill as director for markets intelligence, data and analysis and Andrew Whyte as director of communications within the strategy and competition division.
Wheatley added: “While the FCA has been able to attract some of the best people from outside, we also have highly talented, hard-working people within the organisation and Marc Teasdale, Jo Hill and Andrew Whyte will be taking on roles that will be vital to the organisation successfully delivering its objectives and priorities for the years ahead.”