A longtime donor to Democratic candidates and organizations, Ackman endorsed former President Donald Trump in the 2024 United States presidential election.

At a billed cost of more than $100,000, his law firm copied 725,000 pages of statements regarding the financial services company to comply with a subpoena.

Ackman bought credit default swaps against MBIA corporate debt and sold them for a large profit during the financial crisis of 2007–2008.

They agreed to a "shmuck insurance" arrangement, under which, if Icahn were to sell the shares within 3 years and made a profit of 10% or more, he and Ackman would split the proceeds.

[22] In 2004, with $54 million from his personal funds and from his former business partner Leucadia National, Ackman started Pershing Square Capital Management.

[25] The testifying panel answered questions related to the committee's concerns about repercussions to patients and the health care system posed by Valeant's business model and controversial pricing practices.

[31][32] In December 2012, Ackman issued a research report that criticized Herbalife's multi-level marketing business model, calling it a pyramid scheme.

[33] Ackman disclosed that his hedge fund, Pershing Square Capital Management, sold short the company's shares directly (not with derivatives) starting in May 2012, causing Herbalife's stock price to drop.

Markey later told the Boston Globe that his staff had not informed him that Ackman stood to benefit financially from his actions and defended the letters as a matter of consumer rights.

[41] In March 2014, The New York Times reported that Ackman had employed tactics to undermine public confidence in Herbalife to lower its stock price, including pressuring state and federal regulators to investigate the company, paying individuals to travel to and participate in rallies against it, and boosting Pershing Square's spending on donations to nonprofit Latino organizations.

Mr. Ackman's team also then started to make payments totaling about $130,000 to some of these groups, including the Hispanic Federation — money he said was being used to help find victims of Herbalife.

[42] Later that month, Pershing Square Capital released a 2005 Herbalife distributor training session, in which an employee described high turnover rates and implied that the company's business model was not sustainable.

[46][47] In March 2015, U.S. District Judge Dale Fischer, in Los Angeles, California, dismissed a suit filed by Herbalife investors alleging the company is operating an illegal pyramid scheme.

[53] In November 2017, Ackman told Reuters that he had covered his short-sell position, but would continue to bet against Herbalife using put options with no more than 3% of Pershing Square's funds.

[64] His most notable market plays include shorting MBIA's bonds during the financial crisis of 2007–2008, his proxy fight with Canadian Pacific Railway,[65][66] and his stakes in the Target Corporation, Valeant Pharmaceuticals,[67] and Chipotle Mexican Grill.

[71] Ackman has said that he admires short sellers such as Carson Block of Muddy Waters Capital and Andrew Left of Citron Research.

[72] Critics of Ackman have called him an oligarch, basing accusations on his use of market manipulation and use of his wealth to push for his own policy preferences, particularly with respect to Israel.

[77][78] The donation, along with those of Bruce Berkowitz, founder of Fairholme Capital Management, and Joseph Steinberg, president of Leucadia National, were the three largest individual gifts the center has ever received.

[79] Ackman's foundation donated $1.1 million to the Innocence Project in New York City and Centurion Ministries in Princeton, New Jersey.

[83] In July 2014, Challenged Athletes Foundation, which provides sports equipment to those with physical disabilities, honored Ackman at a gala fundraiser at the Waldorf Astoria hotel in New York City for helping raise a record $2.3 million.

[84] On March 15, 2021, he announced that he donated 26.5 million shares in South Korean e-commerce company Coupang, valued at $1.36 billion, to three entities, one of them his own foundation.

The proceeds would go toward helping "New York's frontline healthcare workers, police and veterans who battle anxiety, depression, addiction and suicide every day", with Ackman matching the winning bid to support the foundation.

[87] In February 2023, Ackman announced that his foundation and an anonymous donor would together fund Sabatini US$25 million over five years to establish and run a new research laboratory.

[90][91][92] On October 8, 2023, following the Hamas-led attack on Israel on October 7, several Harvard undergraduate student groups signed a letter condemning statements from Israeli officials promising to "open the gates of hell" in Gaza, holding Israel's "apartheid regime" "entirely responsible for all unfolding violence", and describing Palestinians in Gaza as being "forced to live in an open-air prison".

"[93][94] In response, Ackman demanded the publication of the names of all students involved in signing the letter so that he could ensure his company and others do not "inadvertently hire" any of the signatories.

[96] Former Harvard president Lawrence Summers called Ackman's request for a list of names "the stuff of Joe McCarthy".

[97] Ackman was involved in a long-standing WhatsApp group chat with Israeli military leaders and top US business leaders with the stated goal to "change the narrative" in favor of Israel by conveying “the atrocities committed by Hamas" and "help [Israel] win the war" on U.S. public opinion following U.S. protests against the Israel-Hamas war.

[101][102] On January 3, 2024, Business Insider published an exposé detailing how Ackman's wife, Neri Oxman, plagiarized portions of her dissertation.

[120] Ackman is a keen amateur tennis player and controversially commented that he could play a close and even doubles match against John McEnroe in a Bloomberg News interview with David Rubenstein in 2020.