The money will fund a 60% expansion in the Alloa brewer's bottling capacity that will let it produce over eight million bottles or 41,600 hectolitres (HL) a year, creating nine jobs.
When it is finished next spring it will consolidate the firm's position as almost certainly the second-biggest craft brewer in Scotland after Fraserburgh-based BrewDog, which currently does 70,000HL a year but is about to expand again too.
Williams is about to announce a substantial increase in sales. According to unaudited figures for the year ended May 1, 2013, sales will be in excess of £4m, an increase of over £1m in two years. This produced a net profit of £217,000.
Scott Williams, managing director and majority shareholder, said the new investment would mean the firm could meet pent-up demand for its brands, which also include Joker IPA, Midnight Sun and Caesar Augustus.
This is partly from UK supermarkets, where the firm sees much room for growth, as it currently relies on Scotland for four-fifths of domestic sales.
It currently has UK coverage for Fraoch with Morrisons and Caesar Augustus with Sainsbury's, while offering several beers through about 100 Tesco stores in England and Wales.
It also recently had three beers selected for Sainsbury's Great British Beer Hunt, a promotion that sees beers from all over the UK given temporary national coverage. Based on sales, Sainsbury's will eventually choose two brands to offer permanently.
Williams said: "At the moment we are struggling to supply the people that want to supply our beer, and there's a lot of export markets that we are not focused on because of inability to meet demand. They are our biggest opportunity for growth."
The firm exports about one-quarter of its beer volumes to 25 different countries, led by Canada and Russia. As well as lifting supply restrictions in these markets, extra bottling power will let it target other promising export markets such as Japan and China.
Williams said that the other benefits of the expansion would be to broaden its third-party bottling business for microbreweries, which accounts for about 30% of its output at present.
Customers include Fyne Ales, Harviestoun, Inveralmond and Deeside, which also uses the Alloa brewer for warehousing and, in some cases, distribution.
"There's lots of new microbreweries opening up all over the place that need a medium-sized bottling facility," said Williams. "We like the fact that we can help them."
The firm's expansion is being financed through a combination of retained profits, a bank invoice advance and a £241,000 grant from the Scottish Government.
Asked whether the brewer had considered using a crowdfunding model akin to BrewDog's Equity For Punks, Williams said he was "not necessarily financially savvy enough to go down that route".
But, he said, he might consider it in future since the brewer had a "good reputation and a great customer base".
He added: "I take my hat off to BrewDog. They're great people and they make great beer."
The expansion will also facilitate a planned move into recycled plastic bottles, known in the trade as PET (polyethylene terephthalate).
He said: "PET lowers your carbon footprint and makes recycling easier. It's kind of a way forward, nobody standing on broken glass.
"I think we'll be the first craft brewer to put beer into PET. Even if people resist them to start with, I'm quite excited about it."
The brewer also intends to introduce a small canning facility at its premises next year.