On Wednesday evening, Henderson held concurrent AGMs and EGMs in Sydney and London that saw shareholders approve a number of resolutions necessary for the merger with Janus.
Janus shareholders gave the same approval to their board in meetings conducted in the US on 25 April.
The approvals pave the way for the official merger of Janus Henderson Investors on 30 May 2017 (31 May in Australia) that will see the company listed on the New York Stock Exchange and the ASX.
Shareholders also approved the delisting of Henderson Global Investors from the London Stock Exchange during Wednesday's EGM.
Speaking to InvestorDaily, Henderson Pan Asia executive chairman Rob Adams said the shareholder approval means "nothing will now stop this merger from occurring".
"There are some regulatory requirements out of the UK about the time between the vote and when we can actually enact some of the things we need to enact. So on the 30 May 2017 we will be relaunching as Janus Henderson Investors," Mr Adams said.
Once Henderson incorporates the assets under management (AUM) of Janus-owned Kapstream Capital, Janus Henderson Investors will manage around $34 billion in Australian AUM, he said.
The asset base for 'Pan Asia' will be approximately $69 billion, and Janus Henderson Investors will manage slightly over $400 billion globally, Mr Adams said.
"We will be one of the largest houses in the world whose total asset base is actively managed," he said – noting that the merged entity will have 2,200 employees globally, 200 across Asia and 70 in Australia.
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