In 1982, he graduated at the University of Hamburg and received the title J. D. After study periods in Florence (Professor Luigi Lombardi Vallauri, filosofia del diritto presso l’Universita degli Studi di Firenze) and Paris, and completing the second state examination in law, Seibert became a judge at the labour court and then at the local court in Hamburg.
In 1986, he moved to the Federal Ministry of Justice in Bonn, where he became head of the division for cabinet and parliamentary affairs under Minister Klaus Kinkel in 1990.
During the 16th legislative period in Germany (October 2005-October 2009), Seibert prepared numerous legislative proposals, such as the Act on Electronic Commercial Registers (Gesetz über das Elektronische Handels- und Genossenschaftsregister sowie das Unternehmensregister – EHUG) and the Act to Modernize the Law Governing Private Limited Companies and to Combat Abuses (Gesetz zur Modernisierung des GmbH-Rechts und zur Bekämpfung von Missbräuchen – MoMiG, 2008) – which constituted the first major GmbH reform since the creation of this law in the late 19th century.
The 17th legislative period (October 2009-October 2013) brought, among other things, the statutory regulation of the partnership company with limited professional liability (PartGmbB).
In the course of the 18th legislative period (October 2013–October 2017), Seibert drew up the politically acclaimed draft on the introduction of a female quota in supervisory boards in the private sector (Law for Equal Participation of Women and Men in Leadership Positions in the Private Sector and Public Service, also known as FüPoG law in force since 1 May 2015).