Baker is a formidable organiser who played a leading role in rebellions against former Prime Minister David Cameron and helms the 80-strong European Research Group.
He has been a constant voice for exiting Europe, and helped to shape the EU referendum by leading a battle to change the rules on purdah, limiting government activity ahead of last year's vote.
Serving as a Brexit minister in his first ministerial role since entering parliament in 2010, Baker will help to replace his fellow Leave supporter David Jones at the department.
Tweeting just hours before the announcement, Baker said the UK should seek the softest exit compatible with controls over laws, immigration, money and trade.
“We need a good, clean exit which minimises disruption and maximises opportunity,” he said.
It also means that the long-time Treasury Select Committee member can now be ruled out from the race to replace committee chair Andrew Tyrie, who stood down at the election.
The appointment continues May's re-structuring of the Brexit department. While secretary of state Davis remains in office, his junior ministers Jones and Lord Bridges have both departed government with negotiations due to begin in just 11 days.
May has also recruited Remain campaigner and peer Baroness Anelay to join the Brexit team.