The WPP-owned agency competed against the incumbent, Brilliant Media, and two other rivals.
Hillarys is the UK's leading made-to-measure blind specialist, with a turnover of more than £85 million and a 23 per cent market share.
About £2 million is spent above the line, with the remainder allocated to inserts and door-to-door activity.
In addition to the main Hillarys brand, MediaCom will plan and buy across webblinds, an online operation launched in 2000, and Arena, which manufactures blinds for other companies. The brief also includes Hillarys shutters and awnings.
The creative business, handled by the Union advertising agency in Leeds, is not thought to be affected.
Launched in 1971, Hillarys employs around 1,000 people in Nottingham and Washington, Tyne and Wear.
MediaCom declined to comment on the news.