The acquisition accelerates the ambition to grow and globalise its business, Henderson said, with Geneva's investment expertise in US growth equities filling what it called an important capability gap.
The addition of US growth equity manager Geneva will double Henderson's number of US-based investment staff.
Post acquisition, Henderson's north American business will have approximately £10.75bn of Assets Under Management.
Henderson told the Stock Exchange: "There is a strong cultural fit between the two firms and Geneva principals will become valued members of Henderson's equity and north American management teams.
"The acquisition will transform Henderson's north American presence, bringing proven institutional distribution capabilities to complement Henderson's successful retail franchise."
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Andrew Formica, chief executive of Henderson, said: "Developing our presence in north America is a strategic priority for Henderson. The acquisition of Geneva is a major step towards achieving our growth ambitions as a global asset manager.
"It will increase our assets under management in the US by over 50%, add investment management expertise in US equities and extend our US institutional client base.
"Henderson clients will gain access to Geneva's strong track record in identifying high quality growth companies in US equities, while Geneva clients will continue to benefit from a stable team and an unchanged investment process, now backed by a global pure play asset manager."
Henderson is an IFP corporate member.
Amy Croen, co-founder and managing principal of Geneva, said: "The team at Geneva is excited to join Henderson.
"With the backing of a strong international partner who is very supportive of our existing investment strategy and platform, we look forward to taking our business to the next stage of its development."