It is unknown when Seyam joined Hamas, but he was one of the early members. Seyam held a job as a teacher at various United Nations-run schools until 2003. He was arrested four times during the First Intifada from 1987 to 1991. In 1992, he was expelled from the Palestinian territories by Israel to southern Lebanon and upon his return in 1995, Seyam was arrested by the Preventive Security Service dominated by Fatah. Seyam was reported by Hamas as one of the pioneer of mortar and Qassam rocket attacks on Jewish settlements in the Gaza Strip, which caused them to be virtually besieged from 2001 to their evacuation by Israel in September 2005. According to his fighters, Seyam was injured three times during that period, although the causes of these injuries were not disclosed. Seyam was the second of Hamas' top five officials to be killed during Israel's offensive. He had evaded at least one attempt by the IDF to kill him in late 2004. Seyam became one of the members of the Hamas "collective leadership" in Gaza after the 2004 killings of founders Ahmed Yassin and Abdel Aziz al-Rantisi. Seyam was close to Yassin and Hamas secretary-general Khaled Mashal, that latter whom he accompanied on a diplomatic visit to Moscow, Russia. After the parliamentary elections in 2006, Seyam was appointed Interior Minister of the Palestinian National Authority, forming the Palestinian government of March 2006. It also put him in charge of 13,000 police and security forces. He represented the Gaza electoral district of the Palestinian Legislative Council as a Hamas member after being elected with the most votes for any single candidate. In June 2007, he created the Executive Force, a paramilitary organization to rival Mohammed Dahlan's PSS. It played a key role in the takeover of Gaza and was praised by many Gazans for bringing order to Gaza, but accused of torture by others.
Said Seyam was killed on 15 January 2009 in an Israeli air raid on his brother's rented home in Jabalia. Also killed in the strike were his son, brother and two other Hamas officials: the interior ministry's security director Saleh Abu Sharkh and the local leader of the Hamas militia, Mahmoud Abu Watfah. Hamas spokesman Sami Abu Zuhri stated "Some of our leaders will fall, some of our people will fall, but the flag of resistance won't fall" in response to Seyam's death. Hamas members stated on Al-Aqsa TV that Seyam's "blood will be the fuel for the coming victory ".
