The Glasgow business is buying Leeds-based disaster recovery firm Backup Technology (BTL) in a deal which could be worth up to £23m.
BTL stores data for the likes of Siemens, British Red Cross, Lloyds Register, Suzuki and Pernod Ricard as well as Liverpool and Everton football clubs.
Iomart is paying an initial £17.5m in cash with a further £3.5m raised through issuing more than 1.2 million new ordinary shares.
A sum of £2m is payable in cash on January 31 next year if certain targets are met.
Mr MacSween said the acquisition gives Iomart access to larger customers. He said: "This takes us into the enterprise market where we haven't really been before.
"It is a good deal as most of the back-up market is still on tape and going around on vans and to warehouses so I think there is a lot of opportunity in the sector.
"We have been tracking [BTL] for two or three years. They are a very lightweight organisation in terms of sales and marketing so we are hoping to inject a wee bit more into that and hopefully get them growing a bit faster than they have been."
BTL, set up in 2005, uses leased data centre space at the moment but Mr MacSween said the business will transfer operations to Iomart's estate once those contracts expire in around one year.
Bank of Scotland is backing Iomart's latest deal with a £35m debt package made up of a £15m loan and £20m revolving credit facility.
Iomart has drawn down £32.5m with £14m used to repay existing bank facilities and the balance used to finance the acquisition.
The net debt of the business following the acquisition will be around £26m.
Mr MacSween said: "We cleaned out the old [financing] facility and introduced a new one. The bank did quite a bit of diligence and seem quite pleased."
Asked about his reasons for selling part of his stake in the business Mr MacSween said he was "sad" about the move as "Iomart is still the best investment I can think of."
However he admitted there had been a desire from investors to create more liquidity in the stock.
He said: "There has been demand for stock and nobody has really been letting any go. There has been quite a lot of pressure on me to provide more liquidity on the stock and that is what drove it more than anything else.
"The other thing is because we are in acquisition mode most of the time we have very few opportunities to trade stock.
"So we have been stuck in the situation for the past couple of years where we have not been able to trade. We have some good new institutions who have come in on the back of it so it has widened our [shareholder] register quite nicely."
Although the exact number of shares and pricing is still to be determined Mr MacSween indicated he is likely to see his stake reduce to around 16%. It is currently more than 19%.
Mr MacSween said he had no plans on how to spend the windfall but was likely to hand much of it to his stockbroker to reinvest. He has agreed not to sell any more shares until after publication of results for the 12 months to March 2015.
Sarah Haran, managing director of Iomart's hosting division, intends to sell up to £1.05m of shares while finance director Richard Logan could offload up to £750,000.
Last month Iomart agreed a deal worth up to £8.1m for Hampshire-based rival Redstation.
It said trading in the current financial year is "strong" with revenue and profits "substantially ahead" of the prior year.
Mr MacSween said: "I look forward to the second half of the year and remain very confident in the growth prospects of the business."
Iomart's previous biggest acquisition was the £12m it paid for Easyspace in 2004.
In August last year it paid £7m for Melbourne Server Hosting in a move with took it into the north-east of England for the first time. It also acquired Surrey web-hosting business Skymarket for £1.4m and Welsh firm Internet Engineering for £1.5m in 2012.
Iomart has around 300,000 customers including broadcaster STV, travel search engine Skyscanner and transport giant Stagecoach.
Shares in Iomart closed down 14.25p, or 5%, at 276p.