On November 25, 1991, Sullivan was appointed by President George H. W. Bush to serve as an Associate Judge of the District of Columbia Court of Appeals.

During the trial, the judge refused requests by the defense for a mistrial to be declared, after information was revealed that the prosecution had withheld exculpatory Brady material.

[20] According to the American Civil Liberties Union (ACLU), a woman and her child fled domestic abuse in El Salvador to seek asylum in the U.S.

However, the mother was removed from her detention facility and likely put on a plane on August 9, 2018, despite Justice Department promises that she and others would not be deported before the judge could rule on their cases.

He threatened to hold those responsible for the removal in contempt of court, starting with Attorney General Jeff Sessions, if the situation was not rectified.

[25] On February 7, 2020, in a per curiam decision, the United States Court of Appeals for the District of Columbia Circuit held that individual members of Congress lacked standing to bring action against the president.

[25][26] The court, finding in favor of Trump, reversed and remanded Sullivan's holding that the members of Congress had standing to sue and instructed him to dismiss the complaint.

[27] The case of United States of America v. Michael T. Flynn, the former national security adviser to Donald Trump, was randomly assigned to District Court Judge Rudolph Contreras.

[47] On May 12, Sullivan announced that he would place a hold on the DOJ's move to drop charges against Flynn, and would consider a hearing involving "friend of the court" (amicus curiae) input from outside parties with interests and perspectives.

[48] On May 13, Sullivan appointed Gleeson as amicus to formally oppose the DOJ's attempt to drop charges, and to look into whether Flynn had committed perjury when he twice told the court under oath that he was guilty.

Circuit Court of Appeals panel granted Flynn's petition for a writ of mandamus, ordering Judge Sullivan to dismiss the case.

"In this case, the district court's actions will result in specific harms to the exercise of the Executive Branch's exclusive prosecutorial power.

The contemplated proceedings would likely require the Executive to reveal the internal deliberative process behind its exercise of prosecutorial discretion, interfering with the Article II charging authority."

[57] In response to the higher court ruling, Judge Sullivan issued an order canceling the July 16 hearing date but did not address the DOJ's motion further.

Judge Sullivan's options were either to request the full appellate court to review the case or to grant the DOJ's motion to dismiss the charges in accordance with the writ of mandamus.

[59] On July 20, 2020, Flynn filed his response to Judge Sullivan's request for the full court to review the panel decision to order the criminal charges dismissed.

[60] By Order dated July 30, 2020, the full court granted Judge Sullivan's request and scheduled oral argument for August 11, 2020.

[61] In addition to vacating the June 24, 2020 panel decision, the Order directs all parties to be prepared to "address whether there are 'no other adequate means to attain the relief' desired" (citation omitted).

[69] After the USPS failure to comply with the court-ordered ballot sweep, Sullivan ordered Postmaster General Louis DeJoy to appear in court.