The German Salafist convert, who had previously sparked outrage by forming a "sharia police" vigilante group, was convicted for helping to recruit for Jaish al-Muhajireen wal-Ansar (JMA) or Army of Emigrants and Supporters.
JMA, which includes Chechens, Uzbeks and Tajiks, first emerged in Syria in 2012, is deemed a terrorist organisation by Germany.
In September 2015, the militants pledged allegiance to the Al-Nusra Front, a rival group of Isis.
Prosecutors said Lau was the main contact for JMA in the western city of Düsseldorf.
The 36-year-old had also travelled to Syria in September 2013 and had purchased night vision equipment for the jihadist group, prosecutors said.
Lau, a former firefighter, had denied the accusations when he was arrested in December 2015.
He argued that his trip to Syria was for humanitarian reasons, despite photos showing him carrying a kalashnikov aboard a tank.
Lau has been a controversial figure in Germany, where he sparked outrage with "sharia police" street patrols in the western city of Wuppertal in 2014, telling people to stop drinking, gambling and listening to music.
However, a German court in November ruled the vigilante group did not break the law.
The city's district court ruled that the seven accused members of the group did not breach a ban on political uniforms when they approached people while wearing orange vests bearing the words "Sharia Police".
Judges said there could only be a violation of the law - originally aimed against street movements such as the early Nazi party - if the uniforms were "suggestively militant or intimidating".
Separately, Germany's anti-terror prosecutors announced on Wednesday the arrest of three suspected Islamists in the northeastern city of Güstrow.
The trio are suspected of having prepared "an act of violence that threatens the security of the state", said federal prosecutors in a statement, without giving details.