The investor, which in October reported a 70 per cent slide in pre-tax profits, said chief executive John Vitalo had stepped down, with a replacement to be announced in "due course".
Vitalo, who joined the company in 2014, was its first ever chief exec.
The news comes shortly after the departure of chairman Arnold Ekpe, who stepped down in October.
Atlas Mara announced some other reshuffles to its senior team today, aside from Vitalo's departure.
Sanjeev Anand, currently managing director at Banque Populaire du Rwanda, will become group managing director for retail and commercial banking. Mike Christelis, Atlas Mara's head of Ttreasury and markets, will be the new group managing director for markets and treasury. Chidi Okpala, Atlas Mara's digital chief, will become group managing director for fintech.
The company also said today it had raised $13.5m (£10.8m) from a placing of 7m new shares, representing just over 10 per cent of the company's issued share capital.
In addition, Diamond, who's been chairman since Ekpe's departure, bought 784,000 shares, while senior managers bought 116,000 - bringing the total amount raised to just over £15m.
The company said it will use the cash to support its operational growth through the rollout and expansion of its global markets and treasury business, scaling up its digital finance business, and for general corporate purposes.
Shares in the London-listed company were down 0.36 per cent at $2.10 in mid-morning trading.