ECI will take a 60% stake in the company, with the remaining shareholding being held by the existing management through significant reinvestment.
The agency, which will retain its independence, was founded in 1999 by Andrew Walmsley and Charlie Dobres.
It now turns over in excess of £100m of online media expenditure across display advertising, search marketing, and affiliate management.
Its clients include the COI, Orange, William Hill, Specsavers and Sony. The Group is split into three divisions: i-level (including Jam), Generator (consulting) and Premier Affiliate Network (PAN).
David Pattison, chief executive of i-level, said: "With the help of Ingenious Corporate Finance we have been looking at how to put the business in the best possible shape for the future.
"With ECI we now have the perfect investor to provide support and a significant war chest for both UK and International investment and acquisition. We are very excited about the future for ilG, its clients and its people, with ECI."
Walmsley said: "We've spent nine years bringing business success to our clients online. The deal with ECI is a great opportunity for everyone at ilG to set this passion for digital on a bigger stage."