He was detained and released after four years, from 2005 to 2009 according to a law he drafted himself, due to his alleged involvement in the assassination of former Lebanese prime minister Rafik Hariri.

Due to the civil war which started in Lebanon early 1975 and led to the division of the Lebanese Army into religious units, Sayyed refused to join any of them.

After the starting of the Israeli withdrawal from Lebanon by mid 1983, Lt Colonel Sayyed was appointed as deputy then chief intelligence officer in the Bekaa Valley Region where he stayed in post until 1991 during which there was two failed attempts in 1985 and 1987 to assassinate him by the then growing Hizbullah under the leadership of Sheykh Sobhi Tfaily whose goal was to occupy the army military bases in the region.

The Surete Generale and its Director Sayyed were praised the same year too by the council of Maronite Bishops headed by Patriarch Sfeir as the only transparent, uncorrupted and efficient Lebanese institution(GDGS Records-UN Report 2004).

(Leb.Army records-2000) As of 2002 Sayyed was assigned by the Lebanese government to head the negotiating team for the exchange of prisoners between Lebanon and Israel with the German mediation which led to their liberation from both sides by January 2003.

(Leb Government Records-2003) Sayyed resigned from office on 25 April 2005[13] as a result of the heavy pressure from the opposition in Lebanon after Rafik Hariri's assassination on 14 February 2005.

[15] A few months after his resignation Sayyed was arrested on 30 August 2005 by a recommendation of the International Commission (UNIIIC) to the Lebanese authorities for his alleged role in the assassination.

In May 2018, Jamil Al Sayyed ran for the 2018 Lebanese general election as an independent candidate to represent the constituency of Baalbek-Hermel at the National Assembly.

He is considered according to Reuters to be a potential successor to Nabih Berri, the Shia speaker of parliament, but this is a difficult step due to the directness of Sayyed that doesn't comfort political parties.