The practice first thought it had landed the £40 million scheme in June, after seeing off stars such as Kengo Kuma to win the much-coveted international contest ( Bureaucrats botch Beamish).
However, the appointment was thrown into doubt when it emerged the museum's funders had decided to vote on whether or not to uphold the competition jury's verdict.
The potential for a change of heart about the winning scheme - a design which was disliked by the outdoor museum's original founder Frank Atkinson - infuriated the RIBA Competitions Office.
In a statement, the Competitions Office said it was 'extremely disappointed by Beamish's delay in appointing Benson + Forsyth' and 'deeply concerned' that it reflected 'unfairly on the selection process.'
Now it seems the joint committee of Beamish The North of England Museum has finally 'ratified' the judges' original decision.
A spokesman for the museum said: 'The decision to appoint Benson + Forsyth is an important milestone for Beamish and its future.'
'We very much look forward to working with Benson + Forsyth to develop the concept and see it through to an inspiring, impressive conclusion.'