Cullen earned a BA in sociology and political science and a MPA from the University of Hawaii at Manoa.

[3] When Karamatsu ran for Lieutenant Governor of Hawaii and left the District 41 seat open in 2010, Cullen won the four-way 2010 Democratic primary with 1,658 votes (43.1%),[4] and won the 2010 general election with 4,510 votes (69.6%) against Republican nominee Carl Wong.

Cullen and Carl Wong, his Republican challenger from 2010, were both unopposed for their 2012 primaries for Oshiro's former seat.

[7] On February 8, 2022, the United States Attorney's District of Hawaii office, on behalf of the U.S. Department of Justice, filed an information charging Cullen and former Hawaii State Senate Majority Leader J. Kalani English with honest services fraud.

[9] Cullen pled guilty to the charge, and was sentenced to 2 years in prison and fined $25,000.