On June 4, 2000, Koster led a group of law enforcement officers in the discovery of the remains of three female victims of Robinson's, each stored in 55-gallon drums inside a Raymore, Missouri storage facility.

[11] During his time in the Missouri Senate, Koster played key roles in the debates over stem cell research, tort reform, and the elimination of Medicaid fraud.

[23][24][5] Koster won the nomination despite accusations from Donnelly and Harris that his campaign violated state law in raising money from multiple committees.

He filed a second amicus brief suggesting that the individual mandate could be upheld under Congress' ability to lay and collect taxes, which is how the Supreme Court ruled on the matter in 2012.

[34] Also in 2012, a report by state auditor Tom Schweich criticized Koster for his practice of awarding contingency fee contracts to law firms that had donated to his campaign.

After the 21 inmates filed suit against the Missouri Department of Corrections over the use of the drug propofol for lethal injections, concerning cruel and unusual punishment, the state Supreme Court temporarily halted the further use of the death penalty until the case was decided.

[38][39][40] During his second term, Koster's office defended U.S. District Judge Catherine D. Perry after she denied a motion for temporary restraining orders on six police officers enforcing a "5-second rule" that required demonstrators to move every five seconds or face arrest in Ferguson, Missouri, citing the need for law enforcement's protection of property and the availability of a designated protest area.

In October 2014, Judge Kimberly Mueller dismissed the suit, ruling that the states lacked legal standing to sue on behalf of their residents and that Koster and other plaintiffs were representing only the interests of egg farmers, rather than "a substantial statement of their populations".

[45][46][47] Also in October 2014, House Speaker Tim Jones announced plans to investigate charges that Koster took actions in office that were designed to benefit campaign contributors.

Despite his position on lobbying reform, Koster rejects the idea of placing limits on the amount of money a corporation or a rich person could contribute to a campaign.

[51][52] During his second term as attorney general, Koster said that he supports same-sex marriage, but defended his state's former constitutional ban on it because voters approved it.