The group said the plunge in AUM to £65.4bn was largely due to "unfavourable market and FX movements" and previously announced sale of New Star Institutional Managers.
Net outflows from Henderson retail in the three months from 1 July to 30 September were £692m, which it said were mainly from the European Sicav fund range.
It blamed uncertainty surrounding the European sovereign debt crisis and market volatility for the resultant decline in risk appetite among investors.
Meanwhile, net outflows from Gartmore retail were concentrated in European and Latin American equity funds.
Total AUM remained higher than the start of the year, as did assets in the separate retail and institutional businesses.
Despite the outflows seen in equity products, the closing AUM of £34bn for that asset class still topped the £30.5bn at the start of the year.
Fixed income products on the other hand failed to recover from the redemptions suffered in the first half of the year and now have £15.6bn under management, compared to £18.3bn on 1 January.
Andrew Formica, the group’s chief executive, said: "Although property and fixed income valuations were broadly unaffected, equity prices fell sharply as investors lowered their risk appetite and switched into cash.
"Investors have taken some comfort from the statement by the eurozone last week but, as evident this week, there are many stages to go through before the package of measures is fully implemented.
"We therefore anticipate uncertain and volatile market conditions for at least the remainder of this year. However, as we saw post 2008/2009, once volatility subsides, investor demand for equities should return."