Schlegelberger was born into a Protestant salesman's family in Königsberg. His father worked in cereal trade sales. His forebears (among them Balthasar Schlögelberger) had been among the Protestants expelled from Salzburg, Austria, in 1731 – 32 and given refuge in East Prussia. Schlegelberger went to the old town Gymnasium in Königsberg, where he passed his graduation examination in 1894. He studied law beginning in 1894 in Königsberg and from 1895 to 1896 in Berlin. In 1897 he sat the state legal examination scoring fairly well.

At the University of Königsberg — or, according to documents from his trial, the University of Leipzig — on 1 December 1899 came his graduation to Doctor of Law with the theme "May government representatives be placed at our disposal as officials because of their voting?"

On 9 December 1901 Schlegelberger took the great state law examination, passing with a mark of "good". On 21 December 1901 he became a court Assessor at the Königsberg local court, and on 17 March 1902 assistant judge at the Königsberg State Court. On 16 September 1904 he became a judge at the State Court in Lyck (now Ełk). In early May 1908, he went to the Berlin State Court and in the same year was appointed assistant judge at the Berlin Court of Appeals (Kammergericht). In 1914 he was appointed to the Kammergericht Council (Kammergerichtsrat) in Berlin, where he stayed until 1918.

On 1 April 1918 Schlegelberger became an associate at the Reich Justice Office. On 1 October of that year, he was appointed to the Secret Government Court and Executive Council. In 1927, he was appointed as Ministerial Director in the RMJ. Schlegelberger had been teaching in the Faculty of Law at the University of Berlin as an honorary professor since 1922. On 10 October 1931 Schlegelberger was appointed State Secretary in the Reich Ministry of Justice under Justice Minister Franz Gürtner and kept this job until Gürtner's death in 1941. On 30 January 1938 Schlegelberger joined the Nazi Party on Adolf Hitler's orders.

Among Schlegelberger's many works in this time was a bill for the introduction of a new national currency which was supposed to end the hyperinflation to which the Reichsmark was prone. After Franz Gürtner's death in 1941, Schlegelberger became provisional Reich Minister of Justice for the years 1941 and 1942, followed then by Otto Thierack. During his time in office the number of death sentences rose sharply. He authored the bills such as the so-called Poland Penal Law Provision (Polenstrafrechtsverordnung) under which Poles were executed for tearing down German posters. Schlegelberg's attitude towards his job may be best encapsulated in a letter to Reich Minister and Chief of the Reich Chancellery Hans Heinrich Lammers:

However, to "spare" some of the half - Jews, he suggested in a letter on 5 April 1942 to Hans Heinrich Lammers, they be given a choice between the "evacuation" or sterilization:

Upon his retirement from the position on 24 August 1942, Hitler gave Schlegelberger an endowment of RM 100,000; in 1944, Hitler allowed him to buy an estate with the money, something that only agricultural experts were entitled to under the rules in force at the time. This would later weigh against him at Nuremberg, for it showed that Hitler thought highly of Schlegelberger.

At the Nuremberg Judges' Trial Schlegelberger was one of the main accused. He was sentenced to life in prison for conspiracy to perpetrate war crimes and crimes against humanity. In the reasons given for the judgment, it says:

‘…that Schlegelberger supported the pretension of Hitler in his assumption of power to deal with life and death in disregard of even the pretense of judicial process. By his exhortations and directives, Schlegelberger contributed to the destruction of judicial independence. It was his signature on the decree of 7 February 1942 which imposed upon the Ministry of Justice and the courts the burden of the prosecution, trial, and disposal of the victims of Hitler’s Night and Fog. For this he must be charged with primary responsibility.

‘He was guilty of instituting and supporting procedures for the wholesale persecution of Jews and Poles. Concerning Jews, his ideas were less brutal than those of his associates, but they can scarcely be called humane. When the "final solution of the Jewish question" was under discussion, the question arose as to the disposition of half - Jews. The deportation of full Jews to the East was then in full swing throughout Germany. Schlegelberger was unwilling to extend the system to half - Jews.’

In 1950 the 74 year old Schlegelberger was released owing to incapacity. For years afterward, he drew a monthly pension of DM 2,894 (for comparison, the average monthly income in Germany at that time was DM 535). Schlegelberger then lived in Flensburg until his death.