He worked for Public Opinion Strategies and DuHaime Communications before managing Bill Baroni's 2003 bid for New Jersey State Assembly—the only Republican to oust a Democratic incumbent that year.

[10] Christie's first major announcement after winning the election was the naming of four top staffers, including Stepien as a Deputy Chief of Staff for Legislative and Intergovernmental Affairs.

[15] The turnaround was a result of Stepien's work on Christie's re-election campaign, where he became embroiled in the Fort Lee lane closure scandal to the George Washington Bridge.

[20][21] Judge Jacobson of the New Jersey Superior Court ruled that Stepien was not required to comply with the legislative subpoenas to produce documents since they were written too broadly, and he was also protected under the Fifth Amendment.

[23][24] Two months after being fired, Stepien made a "soft landing" at the data and phone bank giant FLS Connect, which hired him to work on sales and strategy.

[25][26] Stepien's firm, Nassau Strategies LLC, is based out of a residential home on Knob Hill Road in Washington Township, Morris County, New Jersey.

[33] In December 2018, Stepien and White House Director of Public Liaison Justin Clark were named Senior Political Advisors to the President's re-election campaign focusing on delegate and party organization.

[38] After the 2020 election, Stepien, along with two former members of the Trump 2020 presidential campaign, Justin Clark and Nick Trainer, formed a political consultancy firm, National Public Affairs.

[42] On June 13, 2022, Stepien was scheduled to testify in front of the United States House Select Committee on the January 6 Capitol Attack, but said he was unable to appear because his wife was in labor.