Nettv247.com was founded by Ian West, former deputy chairman at Top Up TV, and recently launched its first internet venture 123webtv.com, which has been running in live beta for nearly a month.
Prior to her time at Kangaroo, the planned on-demand service from BBC Worldwide, Channel 4 and ITV, Mackenzie worked for News Corporation with Tata Sky in India and BSkyB in the UK.
As well as being CEO, Mackenzie has also bought into Nettv247.com and taken a stake in the business for an undisclosed sum.
Mackenzie said: "I have been very excited by the internet TV market and can see its huge potential over the next few years. I am pleased to have become a shareholder and CEO of an important business in this developing sector."
West said: "Having successfully launched our first website in live beta a month ago, Lesley's skills are perfect to achieve our growth objectives for www.123webtv.com and our ambitions in this market."
Ashley Highfield left his position as the director of future media and technology at the BBC in April to take over from Mackenzie as chief executive of Kangaroo.
Earlier this week, the Office of Fair Trading referred Kangaroo to the Competition Commission, delaying its launch until 2009.