The firm is handling all "business as usual" pieces of work including property, minor litigation, contracts and employment services.
An external panel – Pinsents plus Linklaters, Eversheds, Berwin Leighton Paisner and Allen & Overy – will be brought in on a case-by-case basis for more complex work, such as mergers and acquisitions.
Chris Vaughan, chief corporate officer at Balfour Beatty, said: "In addition to the substantial cost savings and closer strategic relationship with Pinsent Masons, the management information we receive will enable us to proactively manage legal risk across our UK business."
Pinsent Masons said it was likely some of the work which may have previously been done in England will be conducted from its Scottish offices in Edinburgh, Glasgow and Aberdeen.
Alastair Morrison, head of client strategy at the firm, said: "A key component of this deal is that our client is effectively incentivised to use us on complex and critical work in the UK and internationally.
"We genuinely believe we are breaking new ground by offering a service which is not fundamentally based on the hourly rate."
Pinsent Masons merged with McGrigors in May last year.