Paul was appointed as a lecturer at Monash University in 2002, initially teaching with the National Centre for Australian Studies, before transferring to Politics in 2005. Paul has twice been the convenor of Politics and International Relations in 2007-08 and 2012-16. Currently, he is a deputy head of the School of Social Sciences.

Paul specialises in Australian political history with a particular focus on political leadership and political parties. He is an author and editor of eleven books. These include his acclaimed biography, Keeper of the Faith: A Biography of Jim Cairns (2002); Neither Power Nor Glory: !00 Years of Political Labor in Victoria (2012), which was awarded the Henry Mayer Prize by the Australian Poltical Studies Association for the Best Book on Australian Politics for 2012-13 and the History Publication Award in the 2012 Victorian Community History Awards; and (with Paul 't Hart and James Walter); Settling the Office: The Australian Prime Ministership from Federation to Reconstruction (2016); and The Pivot of Power: Australian Prime Ministers and Political Leadership, 1949-2016 (2017).

Paul has been a frequent commentator on Australian politics in the print and election media, including contributing many op/ed pieces to the Melbourne Age and for a number of years has had a regular segment on ABC Radio Melbourne.

In 2018, Paul was appointed Visiting Cabinet Historian to the National Archives of Australian.