Andrei Kiselev was one of the three managers named on the £215m Baillie Gifford US Growth trust, which only officially launched earlier this year. He had been an employee of Baillie Gifford for a decade.
He joined the firm directly after finishing university.
The lead manager on the trust is Gary Robinson, with Helen Xiong as co-manager. Baillie Gifford have confirmed they will not be replacing Mr Kiselev.
Both Mr Robinson and Ms Xiong are named as co-managers on the £1.4bn Baillie Gifford American fund, which they run alongside Tom Slater who is the head of US equity desk at Baillie Gifford, and Kirsty Gibson.
The American fund is the absolute top performer out of 55 funds in the IA North America sector over the past one, three and five years to 4 June. Over the past year, the fund has returned 34 per cent, compared with 9 per cent for the average fund in the sector.