Pattison's decision to move on follows a deal in May that saw the private equity group ECI take a majority stake in ILG, which valued the business at £45 million.
I-level has kicked off the search for a new chief executive, and Pattison will remain in the role until a replacement has been found. He will then move to a non-executive role at the company while exploring new opportunities.
Pattison was appointed in April 2007 to develop the group, which also includes the Generator Consulting business, ahead of a sale.
He said: "It's been an exciting time to be in the digital space and i-level is a fantastic business. I've enjoyed every minute of it, but I've completed what I came here to do, and feel this is the right time to make this change."