Trevor Parker, chief executive of Eco City Vehicles, has stepped down to pursue other interests.
The supplier of the licensed Mercedes-Benz Vito Taxi has begun the process of recruiting a replacement.
Parker's career had included senior positions at Lookers and Dixon Motors. He joined ECV is April 2013.
In a trading update, ECV said its 2013 new Vito sales were 543 units, compared to 561 in 2012.
Profit before tax from continuing operations and excluding exceptionals improved to £250,000 from 2012's £200,000 loss.
The company said: "Since the beginning of the year new vehicle sales trading conditions have become challenging due to a combination of new entrants into the London taxi market and the recent Transport for London ambiguity regarding future changes to London taxi emissions requirements in 2018."
It is hoping to benefit from a £2.3m VAT reclaim of a previously related party, the proceeds of which would be used to repay a loan from the KPM UK Taxi Discretionary Pension Scheme, which also owns properties on which ECV operates.
Commenting on behalf of the board, John Swingewood said: "I am pleased last year's results show an improvement over the previous year and were broadly in line with the budget. This quarter we are seeing a softening in sales of new Vito taxis and the board has taken a number steps to address this.
"Going forward the board intends to diversify the Group into other taxi products and services in order to reduce its dependence on new London Vito taxi sales. To this end, we continue to grow our own rental fleet, currently based on the superior Vito taxi, to now over 45 vehicles.
"The Board and management team intend to achieve further diversification through acquisition."