Hannegan moved to Fountain County, Indiana, about 1825 where he worked as a farm hand and as a school teacher.

[4] On April 4, 1829, he married Margaret Chambers Duncan in Knox County, Indiana;[5] they had one son, Sellman Key Hannegan, born January 20, 1832.

He was appointed by President James K. Polk United States Minister to Prussia March 29, 1849, and served 1849–1850.

Irate and possibly jealous, King Frederick Wilhelm IV requested that Hannegan be recalled.

[8] He was stationed at Fort Plymouth due to conflict with the local Potawatomi who were being forced to leave Indiana for Osawatomie in the Kansas Territory.

Despite the evidence, Voorhees decided not present the case to the grand jury, and he too was forced to leave Covington.

He was the keynote speaker at a political rally on February 24, 1859, promoting the candidacy of Stephen A. Douglas for president.

[11] Derided and ridiculed for his performance, Hannegan returned to his room where he overdosed on morphine, his body being discovered the following day.