He worked at Fat Albert's in Ottawa (a sub shop) before heading to Toronto, where he became a bank teller and savings supervisor.
Cullen later returned to Ottawa to join Informetrica (an economics consulting firm) as an economist, moving on to the federal Department of Health and Welfare as a policy analyst.
There he joined the Economists', Statisticians' & Sociologists' Association (ESSA – a federal public service union), where he rose to Vice-President.
In the 1985 provincial election, he ran as a Liberal candidate in the riding of Ottawa West but lost to Ontario Progressive Conservative Party incumbent Reuben Baetz, a cabinet minister, by about 3,000 votes.
[3] Cullen again sought the Liberal nomination to run in the Ottawa West riding in the 1987 election, but lost to local lawyer Bob Chiarelli.
[5] In May 1998, he gained national prominence as the only provincial legislator in the country to vote against the Calgary Declaration, as he opposed any document that didn't recognize the paramountcy of the Charter of Rights.
In the run-up to the 1999 provincial election, Cullen was challenged for the Liberal nomination in the new riding of Ottawa West—Nepean by Rick Chiarelli, a Nepean city councillor.
Cullen was elected to represent Bay Ward in newly amalgamated City of Ottawa in 2000, and was re-elected in 2003 and 2006, defeating high-profile right-wing challenger Terry Kilrea.
[11] On August 31, Cullen dropped out of the race, citing low funds, the addition of another left wing candidate (Clive Doucet) and a desire to defeat Terry Kilrea in Bay Ward.
[14] Cullen subsequently won the Ontario NDP nomination in the riding of Ottawa West—Nepean[15] but was defeated on June 12, 2014, by the Liberal candidate Bob Chiarelli.