Career
After in 1906 Hamburg"s new suffrage law (nick-named Wahlrechtsraub, ie suffrage robbery) increased the influence of voters paying high taxes on the expense of others, which Petersen opposed, he joined the newly formed faction of the United Liberals, one of the predecessors of the post-World War I DDP. Petersen became the head of the United Liberals. On 20 April 1918 the Hamburg Parliament elected Petersen a life-long Senator of Hamburg. On 12 November 1918 the Hamburg revolutionary Soldiers" and Workers" Council deposed the Senate of Hamburg, but reappointed senate and senators as acting administration only on November 18.
In this function Petersen continued into the Weimar Republic, until the complete senate resigned on 27 March 1919, thus ending the life-term mandates under Hamburg"s old 1860 constitution.
On 28–30 March 1919 the Hamburg Parliament, first time elected under equal suffrage by men and women of Hamburg, elected a new senate, into which Petersen and six more pre-war senators were reëlected, besides eleven new senators. Petersen gained 103 of 160 votes.
Petersen was reëlected senator in 1921, 1924, 1928, 1931 and 1932. In 1924 his fellow senators elected him First Mayor of Hamburg, thus head of state and of government (president of the senate) - though under the auspices of a primus inter pares regulation -, and reëlected until 1929.
Then Petersen became Second Mayor (deputy mayor) under his successor First Mayor Rudolf Ross, succeeding him again as of 1 January 1932.
On 4 March 1933 he resigned from office as First Mayor and senator, unwilling to execute orders he considered illegal given by Hitler"s new government.