He was previously the magistrate of Pingtung County (1989–1993) and magistrate of Taipei County (1997–2004). His first election as the Taipei magistrate was aided by a split between the New Party and the Kuomintang. His subsequent reelection occurred by a wide margin despite the ability of the Pan-Blue Coalition to present a united candidate, Wang Chien-shien. He was Secretary-General (Chief of Staff) to the Office of the President of the Republic of China under President Chen Shui-bian (2004–2005). After President Chen resigned as DPP chairman following the 2004 legislative elections, he was elected the 10th-term DPP chairman. Following DPP losses in the 2005 municipal elections on December 3, Su announced that he would, pursuant to a pre-election promise, resign from the chairmanship.