Hemsley will remain on the board as an executive director once he's been succeeded by Alex Ham and Ross Mitchinson as joint-chief executives.
"The business is in really good health having just reported excellent half-year results and is well positioned to grow in the future. I intend to play a full part in helping to develop the business further," Hemsley said.
"Alex and Ross have worked alongside me for a number of years now and I have huge confidence in them, their vision for the firm, and their determination and commitment to its continued success."
The announcement came as Numis reported revenue rose to its highest level in the group's history for a half-year period.
Its revenue rose 24.3 per cent to £56.8m in the six months ended 31 March 2016, from £45.7m during the same period a year earlier.
Full story: Numis founder and CEO to step down: City A.M.