RICK Mather, right, the architect responsible for transforming Oxford’s world famous Ashmolean Museum, has died aged 75 after a short illness.

The American-born architect could claim to have left a lasting mark on the centre of Oxford, with his company having overseen major building projects at several colleges.

But he will be remembered for the award winning £61m extension of the Ashmolean, opened by the Queen in December 2009, below.

His work at the museum on Beaumont Street saw the replacement of all but the original 1845 Cockerell Building and doubled the Ashmolean’s display space.

Arranged over five floors with level access, the building provides 39 new galleries including an extensive temporary exhibition space to host ‘blockbuster’ exhibitions.

Born in Portland, Oregon, Mr Mather studied architecture at the University of Oregon, where he graduated in 1961.

He came to the UK to study urban design at the Architectural Association, before founding Rick Mather Architects in Camden in 1973. He was to serve on the council of RIBA (Royal Institute of British Architects) and was a trustee of the Victoria and Albert Museum.

His work was to include the refurbishment and conservation of historic buildings such as the extension of the National Maritime Museum in Greenwich, and in Oxford it included the ARCO buildings at Keble College and the new auditorium at Corpus Christi College.

Rick Mather Architects went on design a new 150-seat theatre at Worcester College Oxford. His firm is currently working on the remodelling of the listed Mansfield College.

Years earlier while working at the Dulwich Picture Gallery he struck up a friendship with Prof Christopher Brown, who was to go on to become director of the Ashmolean. The two men worked closely together over more than a decade after Mr Mather won the competition to produce a masterplan for the Ashmolean.

Prof Brown said: “Rick Mather was a very great architect and here at the Ashmolean we created what I believe is one of Rick’s finest buildings. He had a special genius for the integration of the old and the new.”

Mr Mather, who was based in London, died on April 20. He leaves a partner, David Scrase.

Details of a funeral and memorial service are still to be announced.