Yeung Sum obtained his undergraduate degree at the University of Hong Kong. He gained his master's degree at the University of York in Britain before returning to earn his doctorate from the University of Hong Kong. Yeung Sum has taught at the University of Hong Kong since 1979 and has been a lecturer in the Department of Social Work and Social Administration since 1985.
Yeung was a member of LegCo from 1991 to 1997, and since 1998. In the 1980s, he was the Chairman of Meeting Point, a pro-democracy political group. He had also served as the Vice Chairman of the United Democrats of Hong Kong (UDHK). With the merging of the two groups to form the Democratic Party in 1994, he continued to serve as a Vice Chairman in the party, until he took the present position of Chairman from Martin Lee Chu-ming upon the expiry of the latter's term in 2001.
Yeung represented a relatively moderate faction within the democratic camp, and discontent with him within the DP led to fragmentation of the party. As a result, legislator Chan Wai-yip quit the party when Yeung took the Chairmanship in 2001, and a number of "Young Turks" left the DP to join The Frontier in 2002.
See Also: Politics of Hong Kong