The passive investment means Roadside will remain an autonomous entity and plans 12 to 14 annual theatrical releases in North America, with Lionsgate handling home entertainment and ancillary markets.
Cohen and D'arbeloff have been busy hiring distribution and marketing staff ever since Roadside's three-year arrangement with IDP Distribution ended in May.
The first hint of a partnership with Lionsgate came in early summer when Roadside announced it would handle the domestic theatrical release of the dirty bomb thriller Right At Your Door.
Lionsgate reportedly paid $2m at Sundance 2006 for worldwide rights to Chris Gorak's film, which will be the first Roadside release since Lionsgate came aboard as an investor.
The film is set to open in August and will be followed in September by Marco Kreuzpaintner's sex traffic drama Trade, which has also moved sideways from Lionsgate to Roadside.
Both cases demonstrate that when Lionsgate buys rights on projects it may subsequently deem Cohen and D'arbeloff's company to be the more appropriate domestic distributor.
In certain cases Roadside may acquire worldwide rights or supplement a domestic buy with select international territories where Lionsgate wishes to administer overseas sales and distribution.
'Howard and Eric are terrific people and our investment in them is intended to give them more support,' Lionsgate's president of theatrical films Tom Ortenberg said.
'It will be business as usual for them except to say that with our ancillary support and investment, they will be able to take a couple of bigger swings at the plate.'
'Lionsgate's investment is a fantastic opportunity for Roadside Attractions and provides us with the resources to get to the next level,' co-presidents Cohen and d'Arbeloff added.
'They started in our business, so of course they get it. Lionsgate's investment will allow Roadside to both continue hitting our 'singles and doubles', such as our releases of Sarah Silverman: Jesus Is Magic and Boynton Beach Club, as well as to hit more home runs like Amazing Grace.'
The deal was negotiated for Lionsgate by executive vice president of corporate operations Wayne Levin, executive vice president of corporate development Marni Wieshofer, director of corporate development Mark Holloway, Ortenberg, and the law firm O'Melveny and Myers.
Attorney Greg S Bernstein acted on behalf of Roadside Attractions alongside Cohen and d'Arbeloff.
Upcoming releases include the drama Starting Out In The Evening, with Frank Langella, Bret Morgen's documentary retelling of the 1968 Chicago riots in Chicago 10, and this year's Cannes Directors' Fortnight hit Caramel.