He also worked as news editor on The Guardian newspaper's website.
He is currently a freelance journalist working for public sector magazines.
More recently he was elected as a councillor with the London Borough of Sutton and became deputy mayor two years ago. He has served the Lib Dem-run council for the past four years which has won praise for its green credentials.
Mr Callaghan was selected as the Lib Dem parliamentary candidate for Southampton Test last year and has been campaigning on the key issues in the city. He organised a petition on the condition of the roads and pavements, collecting 500 signatures at a series of street stalls in Portswood, Shirley and Lordshill.
Mr Callaghan has also helped lead a campaign to save Millbrook Library from closure. He led a march to the Civic Centre to present a 600-name petition to the City Council.
Mr Callaghan was also the only parliamentary candidate from Southampton Test to join the anti-fluoride demonstration to Downing Street.