Francesco Romeo, Strangio's brother-in-law, who is also wanted over alleged mafia activity, was arrested alongside him.
Police said they had found a firearm and ammunition at Strangio's home, where he lived with his wife and son, along with fake identity documents and forgery equipment, and a large amount of cash.
In a statement investigators said the fugitive "went out rarely, and then only in disguise".
The arrests were part of a joint operation between Dutch, German and Italian police.
The two men are expected to appear in an Amsterdam court to allow extradition proceedings to begin.
Police believe the shootings in Duisburg were the bloodiest episode in a long-running feud between two rival clans of the 'Ndrangheta, a crime syndicate from Calabria in southern Italy.
The bodies of six clan members, aged between 16 and 39, were found in the boots of two cars near the Da Bruno pizza restaurant on August 15, 2007.
The feud between the two rivals has claimed nearly 20 lives since 1991.