The lifelong Blues fans will link up with the club’s existing directors and chairman Iain McInnes with immediate effect.
Willan is a Pompey Supporters’ Trust member, community shareholder and president.
He said: ‘This is a very proud day for me as a fan who attended his first game in 1960.
‘We must never forget the massive contribution made by the Trust, presidents and many other people in saving the club from extinction, stabilising the finances and securing the support of our fantastic fan base.
‘But now is the time to move forward and to build on what’s been achieved to date.
‘This means achieving the right balance between ambition – returning to the Championship at a minimum – and ensuring the long-term sustainability of the football club.’
Pompey Supporters’ Trust trio Ashley Brown, John Kimbell and Mark Trapani make up the current directors along with Mike Dyer, John Kirk and Christopher Moth.