A business unit is to be set up in the city hall promotional agency, London and Partners, and its French counterpart Paris&Co.
A joint tourism scheme is due to be launched next year, City Hall said.
Mayor Sadiq Khan said "two of the world's greatest cities" had "so much to gain from joining forces".
Prime Minister Theresa May is to trigger Article 50 on Wednesday - beginning formal negotiations on Brexit.
Speaking at the launch, Mayor of Paris Anne Hidalgo said: "Paris and London share common values and willpower."
The "Paris-London Business Welcome Program" will be run by a "small dedicated team" aiming to "encourage and facilitate the flow of trade and investment between the two cities", a spokesman said.
London receives more inward investment from Paris than any other global city, having attracted Â£2.6bn that has generated almost 10,000 jobs in the last 10 years, according to City Hall.
Paris, in comparison, is the largest European destination for foreign direct investment from London, with more than 160 London-based companies set up in the city since 2006, creating 7,500 jobs.
And, according to figures released by the partnership, tourists spend in excess of Â£30bn and support up to 1.2m jobs in both London and Paris.
A London and Partners spokesman told the BBC it was hoped similar schemes would be rolled out "to other European and world cities".
The mayors of London and Paris have announced joint tourism and business schemes, a day before the UK's formal exit from the EU is triggered.