Ralph Bernard, currently the executive chairman, will replace him.
Mansfield, previously the chief executive of Capital Radio, has shared the top job at GCap since the newly merged company's inception in May.
He has sat alongside Bernard, the former GWR executive chairman.
Mansfield has overseen the day-to-day operations of the company, while Bernard has covered digital strategy and negotiations with regulators.
However, shareholders have complained about the structure. In May, it was reported that major investors wanted either Bernard or Mansfield to step down.
Four months on, Bernard will take sole charge of the company and his executive chairman role will be abolished. Peter Cawdron has been reappointed as the non-executive chairman, a position he held at Capital Radio before the merger.
Cawdron said: "Having two senior executives made a lot of sense in the months following the merger. However, with the early integration nearing completion and the group's strategy review well under way, both David and Ralph felt the time was right to review the structure and together agreed this way forward."
Mansfield, who worked as Capital Radio's chief executive for eight years, will step down as chief executive of GCap immediately. However, he will stay at the company until January to oversee the final stages of the post-merger integration. It is unclear what he will do after this.
GCap is releasing a trading update on 28 September and interim figures in November.
In the most recent Rajar figures, Capital 95.8 FM recaptured the top spot in London from Heart 106.2 by maintaining a 6.1 per cent audience share. Its audience slipped by 169,000 to 1.9 million, but it still outperformed its main rivals, Heart and Magic 105.4.