"This acquisition is central and complementary to our strategic objective to grow our market share of new mortgage lending."
First Mortgage was established in Scotland over two decades ago and has now commenced its expansion south of the border with a team of 90 mortgage and protection advisers.
In 2018, First Mortgage arranged approximately £2 billion in mortgage lending.
In the nine months ended 31 December 2018, First Mortgage achieved revenue of £10.2m and profit before tax of £1.5m, with around half of its revenues derived from protection products
MAB says the acquisition will present it with "significant additional growth opportunities" to expand adviser numbers and market share.
MAB has also entered into an option agreement to acquire the remaining 20% of First Mortgage between 2024 and 2030 for between £4m and £10m.
The acquisition is expected to complete on the 1st of July 2019.
Ben Thompson, managing director of Mortgage Advice Bureau, said: "This acquisition is central and complementary to our strategic objective to grow our market share of new mortgage lending. First Mortgage is a highly reputable, forward thinking business of significant scale with a clear strategy to deliver further strong year on year growth. This enables us to pursue at pace our aim to accelerate Group growth through excelling in areas of specialisation, and in particular, building on our omni-channel choice for customers.
"This approach best positions MAB to benefit in an evolving intermediary sector. MAB's new technology developments will further strengthen our capability and ambition in this regard."