Hackenburg graduated from New York Law School, after which he began practicing in New York City and had an office at 261 Broadway. Connected with the Lenox Hill Settlement since 1907, he was president of the Intersettlement Debating League and counsel for the Lenox Hill Tenants' Association. In 1920, he was elected to the New York State Assembly as a Democrat, representing the New York County 14th District. He served in the Assembly in 1921,[3]1922,[4]1923,[5]1924,[6]1925,[7]1926,[8] and 1927.[9] He was a member of the State Industrial Survey Commission from 1926 to 1927.[2] He became well known for his liberal perspective in the Assembly, fighting for a revision of labor laws and the "oppressive Sabbath laws" and seeking to do away with "bootlegging in child labor." His efforts won the support of the Citizens Union and the friendship of future Mayor Fiorello La Guardia.[10]
Hackenburg was La Guardia's first judicial appointment, receiving the appointment on La Guardia's first day as mayor. He was appointed to the Special Sessions Court to fill the expired term of Justice William T. Fetherson.[11] He previously broke from Tammany Hall and supported La Guardia, serving as a behind-the-scenes director for his mayoral campaign. In 1935, La Guardia named him one of the nine members of the New York City Charter Revision Commission, whose report a year later became the basis for the restructuring of the New York City government. As Justice, he did a year and a half study of 7,500 paternity cases brought before the Special Sessions Court, which helped bring order to those cases and made him the court's specialist in that field.[11] He was reappointed to the Court in 1944, and he continued to serve as Justice until the day he died.[2]
Hackenburg's wife's name was Julia, and his children were Frederick Jr. and Mrs. Janice Flath. His son Frederick became associated with his law practice, Hackenburg & Schwartz.[10]