The group said on Monday it had set a price range of 190 to 235 pence per share, valuing itself at £1.75 billion to £2.1 billion.
Global stocks fell 10.2% last week as concerns about the impact of coronavirus escalated, prompting speculation that stock exchange floats would have to be pulled.
"In spite of the current backdrop of market volatility and uncertainty, we remain committed to the execution of this transaction, because of its long-term benefits," Hendrik du Toit, founder and chief executive of Ninety One, said in a statement.
The planned global offer in London and Johannesburg is expected to raise £181.9 million to £226.1 million, Ninety One said.
Investec manages £121 billion in assets and announced plans to split off its asset management business last year, copying similar demerger moves by Prudential, Old Mutual, and Deutsche Bank as fees fall and costs rise in the fund management sector.
A prospectus with full details of the offer is expected to be published later on Monday.
Ninety One said it intends to have a free float representing 60-65% of its combined total issued share capital. The company expects to be included in several stock indexes, including the FTSE UK Index Series.
Ninety One said it had net inflows of £3.2 billion and an operating profit before exceptional items of £97.3 million for the six months ended 30 Sept. 2019.
(Reporting by Huw Jones; Editing by Rachel Armstrong)