Before holding positions in the Reagan and senior Bush administration, Roberts served as a law clerk for Judge Henry Friendly and Justice William Rehnquist.

[9] As chief justice, Roberts has authored majority opinions in many landmark cases, including National Federation of Independent Business v. Sebelius (upholding most sections of the Affordable Care Act), Shelby County v. Holder (limiting the Voting Rights Act of 1965), Trump v. Hawaii (expanding presidential powers over immigration), Carpenter v. United States (expanding digital privacy), Students for Fair Admissions v. Harvard (overruling race-based admission programs), and Trump v. United States (outlining the extent of presidential immunity from criminal prosecution).

[24] One of Roberts's first papers, "Marxism and Bolshevism: Theory and Practice," won Harvard's William Scott Ferguson Prize for the most outstanding essay by a sophomore history major.

A recent surplus of history graduate students convinced him to attend Harvard Law School for better career prospects, though he maintained his original goal to become a professor.

At the end of his clerkship with Rehnquist, Roberts worked to gain admission to the bar, studying with Michael W. McConnell, a law clerk of Justice William Brennan.

"[41] Among those he worked with were William Bradford Reynolds in the Civil Rights Division, former classmate Richard Lazarus, J. Harvie Wilkinson III, Theodore Olson, and fellow special assistant Carolyn Kuhl.

[55] Thomas Merrill, a deputy for the Solicitor General, described Roberts's candid position simply as: "This affirmative action program violated the Constitution, and we should present that to the Supreme Court.

[63][64] In June 1995, to Roberts's satisfaction, the Supreme Court overruled his previous loss of Metro Broadcasting, Inc. v. FCC in Adarand Constructors, Inc. v. Peña, establishing that the government must treat people on an individual basis.

[72] Luttig, Wilkinson, and other Reagan officials were leading candidates, but Judge Alberto Gonzales of the Texas Supreme Court, a close supporter of Bush, also emerged and had a chance to be the first Latino nominee.

[73] Roberts, who had not worked in government while Bill Clinton was in office, did not appear on lists compiled by Bush supporters, advocacy groups, or the media, but nonetheless remained a strong candidate for a Republican nomination and was poised to be re-nominated to the D.C.

[80] Even when Roberts had not yet fully assumed his role as a circuit judge, White House Counsel officers listed him on their shortlist of Supreme Court candidates.

[87] By the time of the 2004 presidential election, Justice Rehnquist had been fatally ill and senior Bush administration advisors under Karl Rove began assessing the potential candidates to replace him.

[88] On July 19, 2005, President Bush nominated Roberts to the U.S. Supreme Court to fill a vacancy to be created by the impending retirement of Justice Sandra Day O'Connor.

"[107] Seventh Circuit judge Diane Sykes, surveying Roberts's first term on the Court, concluded that his jurisprudence "appears to be strongly rooted in the discipline of traditional legal method, evincing a fidelity to text, structure, history, and the constitutional hierarchy.

[111] In November 2018, the Associated Press approached Roberts for comment after President Donald Trump called a jurist who ruled against his asylum policy an "Obama judge."

[128][129] On July 9, 2020, Roberts wrote the majority opinion in Trump v. Vance, regarding presidential immunity from criminal subpoenas relating to the president's personal information.

[130] In doing so, he rejected arguments relating to the investiture of absolute immunity in either the Supremacy Clause or Article II of the Constitution or of presidential entitlement to a higher standard of issuance of a subpoena.

"[142][143] Roberts wrote the opinion in the 2007 decision FEC v. Wisconsin Right to Life, Inc., which held that provisions of the Bipartisan Campaign Reform Act of 2002 that limited political advertising were unconstitutional as applied to WRTL's issue ads preceding the election.

[146] Roberts wrote the plurality opinion in the 2014 landmark campaign finance case McCutcheon v. FEC, which held that "aggregate limits" on the combined amount a donor may give to various federal candidates or party committees violate the First Amendment.

Kennedy, writing for a five-justice majority, distinguished Stenberg v. Carhart, and concluded that the Court's previous decision in Planned Parenthood v. Casey did not prevent Congress from banning the procedure.

[156] Roberts also joined with liberal justices in 5–4 decisions temporarily blocking a Louisiana abortion restriction (2019)[157] and later striking down that law (June Medical Services, LLC v. Russo (2020)).

[162] In September 2021, the Supreme Court declined an emergency petition to temporarily block enforcement of the Texas Heartbeat Act, which bans abortion after six weeks of pregnancy except to save the mother's life.

"[172] Roberts cited these cases in writing for the Parents Involved majority, concluding that the school districts had "failed to show that they considered methods other than explicit racial classifications to achieve their stated goals.

Writing for an 8–1 majority, Roberts found that a federal statute criminalizing the commercial production, sale, or possession of depictions of cruelty to animals was an unconstitutional abridgment of the First Amendment right to freedom of speech.

[176] On March 2, 2011, Roberts wrote the majority opinion in Snyder v. Phelps, holding that speech as a matter of public concern, even if considered offense or outrageous, cannot be the basis of liability for a tort of emotional stress.

"[181] On June 28, 2012, Roberts wrote the majority opinion in National Federation of Independent Business v. Sebelius, which upheld a key component of the Patient Protection and Affordable Care Act by a 5–4 vote.

[191] In October 2020, Roberts joined the justices in an "apparently unanimous" decision to reject an appeal from Kim Davis, who refused to provide marriage licenses to same-sex couples.

[194][195][196] In November 2021, Roberts voted with the majority in a 6–3 decision to reject an appeal from Mercy San Juan Medical Center, a hospital affiliated with the Roman Catholic Church, which had sought to deny a hysterectomy to a transgender patient on religious grounds.

[200][201][202] In Shelby County v. Holder (2013), he struck down requirements that states and localities with a history of racially motivated voter suppression obtain federal preclearance before making any changes to voting laws.

[82] During the late 1990s, while working for Hogan & Hartson, Roberts served as a member of the steering committee of the Washington, D.C., chapter of the conservative Federalist Society, although he has said he has little recollection of any involvement.