He rose through the ranks of the department, ultimately becoming superintendent in August 2014, appointed by Mayor Mitch Landrieu to replace Ronal Serpas.

Despite this, Harrison ultimately agreed to join the BPD in January 2019 after leading candidate Joel Fitzgerald dropped out.

He became Commander of the Specialized Investigations Division of the Public Integrity Bureau, which manages the narcotics, vice, criminal intelligence, and gang enforcement units, in January 2011.

[4][2][6] In 2018, while he was superintendent, New Orleans saw 146 homicides, the lowest figure since 1971, a 28% drop in lethal shootings, while car robberies and thefts had increased in the double digits.

Mayor Catherine Pugh chose Joel Fitzgerald, the chief of the Fort Worth Police Department, to become commissioner.

Fitzgerald, however, withdrew his candidacy due to resumé issues and a medical emergency involving his son,[6] and Harrison agreed to become acting commissioner in January 2019.

[10][11] On June 6, 2023, during a budget hearing, Baltimore City Council member Eric Costello asked Harrison if he planned to stay for the entirety of his term as police commissioner.