The Glasgow-based private equity investor believes R&M Engineering could enjoy years of strong growth amid booming investment in the North Sea.
Based in Huntly, Aberdeenshire, R&M Engineering provides services like the design and fabrication of modules and pipework for oil rigs that are being refurbished or for new facilities.
Maven invested in R&M Engineering following a big increase in sales at the firm, which has been capitalising on efforts of oil and gas firms to ramp up North Sea production to meet strong demand.
This has involved companies approving record levels of spending on maintaining ageing infrastructure in the area and developing new assets.
Jock Gardiner, partner at Maven, said the firm invested in R&M Engineering in the expectation this market will remain buoyant for years.
"With a combination of upgrade work and new development it's going to remain strong," he said.
The private equity firm believes R&M Engineering could speed up its growth by widening the range of services it provides.
Part of the funding will be used to help the company develop a survey and imaging service that will be able to produce accurate 360 degree images of sections of rigs.
The company currently sub-contracts this kind of work to other firms. There is scope for R&M Engineering to win more work in overseas markets.
While most of the company's income is generated in the North Sea, some of the parts it produces in Scotland are used on oil and gas facilities on the other side of the world.
Mr Gardiner said R&M Engineering has a blue chip customer base.
This includes firms that operate oil and gas fields and leading services companies.
While the focus will be on organic growth, Maven could support bolt-on acquisitions by R&M Engineering.
The investment is the latest in a series of Scottish oil and gas services deals that Maven has completed in recent years.
In August the firm supported the acquisition of the Ross Offshore engineering operation by a portfolio company Glacier Energy Services.
In June Maven led a £9m investment in HCS Control systems, which specialises in subsea systems, with the Simmons Parallel Energy Fund.
The latest accounts for R&M Engineering filed at Companies House show the company increased turnover to £13.8m in 2012, from £8.1m in the preceding year.
The company made a pre-tax profit of £1.8m in 2012, after losing £78,000 in the year before.
It employs around 70 people.
Maven did not provide details of the investment in R&M Engineering, which was established in 1979 by Mr Mackay.
It is understood the bulk of the funding will be used to develop the business. Some went to existing shareholders.
Mr Mackay had a controlling stake prior to the deal.
He will remain on the board as a non-executive director.
Alan McLean, who held the balance of the shares before the deal, will continue to lead the management team.
Scotland's oil services sector has attracted strong interest from private equity firms with headquarters in London or overseas, including the US and Norway.