[1] Rosengart is known for representing high-profile clients, including Steven Spielberg, Sean Penn, Julia Louis-Dreyfus, Kenneth Lonergan, Eddie Vedder, Jimmy Butler, Sandton Capital Partners,[2] Verizon, and Facebook.

[13] Samuel Gentile of Clearwater, Florida, was sentenced to more than 46 years for attempting to kill his estranged wife with a pipe bomb.

Rosengart conducted a money laundering investigation and obtained a felony conviction against Riady, and secured an $8.6 million fine—the largest campaign finance fine at that time.

[15][5] The head of the Campaign Financing Task Force was now U.S. District Judge Robert J. Conrad, who said: "Matt Rosengart is one of the sharpest lawyers I've ever met at the Department of Justice" whose "judgment is impeccable.

[1] Rosengart has worked on numerous high-profile cases, including those involving Britney Spears, Sean Penn, Winona Ryder, Julia Louis-Dreyfus, Keanu Reeves, Daymond John, and Steven Spielberg.

"[24] Mathew Rosengart successfully defended Kenneth Lonergan, an Oscar-winning writer, and director, from a lawsuit filed by Gary Gilbert.

Gilbert was the producer of Lonergan's critically acclaimed film, Margaret, starring Matt Damon, Mark Ruffalo, and Anna Paquin.

[26] Lonergan attributed the outcome of the case to Rosengart, saying in an interview, "After five years of expensive and highly contentious litigation, the plaintiff suddenly dropped all of his claims in the middle of the trial, without any guarantee of ever receiving a dime.

Rosengart obtained and filed in New York Supreme Court an Affidavit from Madonna confirming that Penn had never physically assaulted her.

[28] Rosengart negotiated a settlement on Penn's behalf, which involved the actor dropping the lawsuit in exchange for a retraction, an apology, and a charitable donation from Daniels.

"[1] Rosengart represented Raven Capital Management an asset-based opportunity fund in their dispute with Green-Light International Films.

The arbitrator, Jeffrey G. Benz, faulted Green-Light principals for lying under oath, failing to produce bank statements, and violating a distribution agreement.

[31] Sandton Capital Partners, a hedge fund, hired Mathew Rosengart to defend a lawsuit against Gold Circle Films.

The lawsuit was resolved in 2019 after a trial when New York Supreme Court Judge Barry Ostrager ordered Gold Circle Films to repay the balance of the loans, which equaled $14 million.

[35] California government entities alleged that the telecommunications carriers failed to meet contractual obligations concerning affordable cellphone plans.

Rosengart's petition alleged Jamie had committed multiple acts of mismanagement and that his suspension or removal was in the best interests of Spears.

[40] The petition came on the heels of Spears's accusations against her father on June 23, 2021,[41] alleging that Jamie has been abusing his authority and taking financial advantage of her.

[44] On August 12, 2021, in response to Rosengart's petition, Jamie filed paperwork with the Los Angeles Superior Court to remove himself as conservator.

[46] Rosengart expressed his approval for Jamie's decision to step down but indicated that he may still face legal repercussions for profiting from Spears's estate.

[46] On September 22, 2021, Rosengart filed a supplemental petition calling for the immediate resignation of Mr. Spears as the conservator of the estate, stating "While the entire conservatorship is promptly wound down and formally terminated, it is clear that Mr. Spears cannot be permitted to hold a position of control over his daughter for another day.” [50] Rosengart also requested a hearing on the continued mismanagement of Spears's estate by her father and the outstanding accounting and financial issues surrounding the conservatorship, citing “unwarranted commissions from his daughter’s work, totaling millions of dollars.” [50][51][52] On September 29, 2021, Judge Brenda Penny granted Rosengart's petition to suspend Jamie Spears immediately from his position as his daughter's conservator.

"[57][58] When asked if Britney would perform again, he stated, “it’s up to her.” [59] In 2023, Rosengart was noted as a "Key Figure" in Spears' book deal with Simon & Schuster reportedly valued around $15 Million.

[78][79] In September 2022, he was honored with the Excellence in Advocacy Award by the Beverly Hills Bar Association, and Law360 recognized him as Media & Entertainment MVP.

[4][84][85] Rosengart ranks among the top litigators in the U.S., having been recognized by The Best Lawyers in America in 2023 and 2024 for his work in Criminal Defense: White-Collar.