Johnson, who was part of the team that saw Jupiter float on the London Stock Exchange in June 2010, said it was a privilege to play a role in the company’s transition from a private firm to a FTSE 250 constituent.
The company has made a number of promotions this year, resulting in the "narrowing" of the CFO role, the company said.
Johnson will stay on until 30 May as Jupiter looks for a replacement.
“I am looking forward to my next challenge, but can look back with pride on what has been achieved at Jupiter. The business is in an excellent position and I wish it every success going forward,” he said in a statement.
In September Jupiter promoted Stephen Pearson to the role of chief investment officer. It has made a series of promotions to the executive committee, including those responsible for fund operations, HR and investment operations.
Shares closed down 0.38 per cent to 470.60p.