The combined business will manage £1.8 billion in discretionary private client assets and platform based model portfolios.
The deal has received FCA approval and is scheduled to complete on Wednesday 30 April.
Ingenious was established in 2003 and specialises in managing money on behalf of individuals, charities, families, trusts and pension funds. It manages £1.5 billion of client assets, primarily in global multi-asset portfolios, and over £120 million on platforms.
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Also established in 2003, Thurleigh manages £300 million of on and offshore assets on behalf of its clients, including private investors, family offices and charities
David Rosier, co-founder and chairman of Thurleigh Investment Managers, said:
"We are delighted to be joining forces with Ingenious Asset Management, which offers a strong cultural fit for our clients together with a stimulating working environment for our team. Most importantly, from our clients' perspective this move offers continuity and the advantages of a deeper pool of investment expertise."
Guy Bowles, founder and chief executive of Ingenious Asset Management, said:
"I have known David and Charles for a number of years and have always admired the business they have built.
"I am excited about the prospect of merging our firms and believe clients of both businesses will benefit from the knowledge and expertise that they and their team bring to us."