The deal come after after Mouchel Rail was sold to Australian consultant Sinclair Knight Merz for £3.4m.
Mouchel will use money from the sale – which could reap a total of £3.5m in total – to repay some of its debt pile.
The unit has clients including National Grid.
Mott MacDonald chairman Keith Howells said: “Mouchel’s gas pipeline team shares our culture of professional excellence, innovation and environmental Sustainability.
This acquisition means we will be able to offer our clients an enhanced suite of capabilities embracing all areas of the energy sector and all facets of project development.”