He passed the first state law examination in July 1911 and began his training as a Referendar (lawyer trainee) at the district court in Warendorf.

At the outbreak of the First World War, he reentered the Imperial German Army as an officer candidate and fought on the western front.

[3] On 8 March 1933, Wilhelm Frick, the Reich Interior Minister appointed Matthaei as the Reichskommissar for the Free State of Schaumburg-Lippe and entrusted him with the exercise of police powers.

[6] Due to disagreements with the Party leadership of Gau Westphalia-North over his positive attitude toward Christianity, on 21 October 1934 he was removed from his post in Münster and transferred to become the president of the Lüneburg District on 1 November.

[7] During the Second World War, Matthaei was assigned as a special representative of the leadership staff of Kiev (today, Kyiv) in the Reichskommissariat Ukraine from July 1943 until March 1944 when he retired from his posts and settled in Dannenberg.

He joined the neo-Nazi Socialist Reich Party (SRP), became a district leader and unsuccessfully ran for a seat in the Landtag of Lower Saxony.

In the Bundestag election of September 1953, despite being considered the strongest candidate that they fielded, he was soundly defeated for a seat from Lower Saxony.