Plummer represents the 55th District, which is located on the Illinois side of the Greater St. Louis region and includes all or portions of Bond, Clay, Clinton, Effingham, Fayette, Madison, Marion, Montgomery, Richland, Washington, and St. Clair counties.

[5] During college Plummer interned for United States Senator Peter Fitzgerald of Illinois and, in Washington, DC, at The Heritage Foundation.

[6]  He commissioned as an Intelligence Officer in the United States Navy Reserve after college and spent time at Scott Air Force Base among other locations.

[13] During his time in the Illinois Senate, Plummer was named Legislator of the Year for 2022 by the Associated Builders and Contractors[16] and received the St. Louis Regional Chamber's Catalyst Award.

[22] Bills sponsored/cosponsored by Plummer that were signed into law during the 101st General Assembly covered a wide-range of topics, with several focused on enhancing penalties for child pornography and sexual assault, as well as enhancing protections for victims of such crimes,[23][24] re-writing the Uniform Code of Military Justice in Illinois to better conform with the federal UCMJ, lessening regulations and fees on business,[25] helping the developmentally disabled, and preserving the Kaskaskia Watershed,.

[27] During the COVID-19 pandemic in 2020, Illinois requested over $40 billion in federal bailout funds to address its significant budget deficits and unfunded pension liabilities.

[28] Senator Plummer opposed this request, arguing that federal funds should not be used to rescue the state from its longstanding fiscal mismanagement and corruption issues.

[32] According to media reports, Plummer had openly pressured Brady to pursue more stringent ethical reforms in private Senate Republican meetings, but the Minority Leader opted to embrace "low-hanging fruit" offered by Democrats.

[33][34] Plummer, a first term senator at the time, received favorable commentary from other legislators and the media for his actions and subsequently filed a package of bills focused on ethics and corruption.

These included Veterans-related issues, combating the opioid epidemic, imposing stricter measures against countries with non-market-based economies, increasing penalties for human trafficking, child pornography, and sexual assault, enhancing protections for victims, reducing regulations and fees for businesses, and supporting individuals with developmental disabilities.

[41][42] He brought similar legislation back during the 102nd General Assembly, and it passed both chambers unanimously, making Illinois the 3rd state in 150 years to rescind their ratification of the Corwin Amendment.

[43] Plummer worked with several historians to address this complicated topic and sought and received support from members of the Illinois Legislative Black Caucus.

[46] After multiple staff members were hospitalized due to possible toxin exposure at the Graham Correctional Facility,[47] Plummer called for transparency regarding the incident.

[56][57] Plummer called for accountability, writing op-eds,[56][57] holding press conferences,[58] and pushing for Senate votes on unconfirmed appointees.

He was formerly a partner and senior adviser at Cimbria Capital, a private equity firm focused on water-related issues and headquartered in Houston, TX.

[73][74][75] Plummer also is involved in the development, management, and ownership of a large portfolio of retail, office, industrial, agricultural, and hospitality properties across the United States.

Lumber Center, an $11.2 million[81] recreational complex containing a regulation ice hockey rink, indoor track, and other amenities, opened in June 2022 following a donation from the Plummer family.