Yearsley joins from Credit Suisse where he spent 23 years and was head of pricing & valuations and pooled pension administration.
He will report to managing director Ian Chimes.
Yearsley’s appointment comes as PSigma takes on the management of the four former Credit Suisse income funds totalling £600m in assets.
Chimes says: “With assets under management now exceeding £1 billion we are ensuring that the business is robustly resourced. As sub-investment adviser to Premier on four of the Credit Suisse funds it purchased from Aberdeen, Lachlan is completely familiar with these funds as they transition to us at PSigma.”