He taught at the City University of Hong Kong (CityU) since 1989 and became Senior Lecturer at the School of Continuing and Professional Education.

[3] He joined the Hong Kong Professional Teachers' Union (HKPTU), the largest teachers' union in the territory, in 1989 and was invited by president Cheung Man-kwong to sit on the PTU executive committee after the success of the "Save the College Action Group of CityU" action group.

In 2010, Fung was elected as the convenor of the Alliance for Universal Suffrage, a coalition consisting of eleven pro-democracy parties on the issue of the government's reform package on the 2012 elections of the Chief Executive and the Legislative Council to be the balanced force in the midst of the increasing differences and conflicts on the approaches on striving for universal suffrage between the moderate faction led by the Democratic Party and the radical faction League of Social Democrats (LSD).

Fung later endorsed the Democrats' revised constitutional reform package and therefore was heavily criticised by the LSD chairman Wong Yuk-man.

In 2013, Fung announced the Alliance "will be indefinitely suspended" in order to form a new body with other pan-democrats on the matter of full universal suffrage.