Elon Musk Seems to Answer to No One. Except for a Judge in Delaware.

Judge Kathaleen St. J. McCormick has become a very important person in the rambunctious life of Elon Musk.

The Delaware Chancery Court judge has given Mr. Musk until Friday to close his long-promised, $44 billion deal to acquire Twitter. If he doesn’t, Judge McCormick will preside over a trial in November that could end with Mr. Musk being forced to make good on the deal he made with Twitter in April.

The 43-year-old judge is also expected to preside over another case involving Mr. Musk in November. A Tesla shareholder accused him in a lawsuit of unjustly enriching himself with his compensation package while running the electric vehicle company, which is Mr. Musk’s main source of wealth. The package, which consisted entirely of a stock grant, is now worth around $50 billion based on Tesla’s share price.

Judge McCormick is also overseeing three other shareholder lawsuits against Mr. Musk, though it is not yet clear whether those will go to trial, too.

The woman who suddenly has a great deal of influence over Mr. Musk comes from a much different world than the jet-setting, South African-born billionaire. The daughter of a high school football coach and an English teacher, Judge McCormick was raised in Smyrna, Del., a town with roughly 13,000 people about 14 miles away from Dover, the state capital.

Judge McCormick now oversees the 230-year-old court that is considered the foremost destination for adjudicating disputes over mergers and acquisitions and other corporate disagreements. She has been both quick-witted and blunt in months of hearings for Twitter’s lawsuit. And her decision to grant Mr. Musk a delay to a trial that was expected to begin earlier in October also displayed unusual flexibility — and pragmatism — to legal experts.

“I was surprised,” said Ronald Barusch, a retired mergers and acquisitions lawyer who worked for the law firm Skadden Arps before it represented Mr. Musk. But, he said, “the deal will either close and then she will be a hero. Or not and Musk will look really bad.”

As a young girl, Judge McCormick played first base on the softball team and managed the high school football team. She has a long-held soft spot for the book “To Kill a Mockingbird,” about a Black man in small-town Alabama who was wrongfully accused of sexual assault.

After graduating from Smyrna High School, Judge McCormick received an undergraduate degree in philosophy from Harvard University and her law degree from Notre Dame Law School.

She then worked as a staff attorney with the Community Legal Aid Society, where she represented the needy and victims of domestic violence. She moved to a corporate law role at the firm Young Conaway Stargatt and Taylor in 2007, a mainstay in the Delaware legal circuit.

In 2018, she was nominated by John Carney, the governor of Delaware, to serve as vice chancellor on the state’s high court, the Delaware Chancery Court. In 2021, Gov. Carney nominated Ms. McCormick to become the first woman to lead the court.

More than 1.8 million businesses are incorporated in Delaware, including more than two thirds of Fortune 500 companies — and they all look to the court for guidance. When Twitter filed its lawsuit against Mr. Musk in July forcing him to close his acquisition, its case went to Delaware, where the company, like many others, is incorporated.

Judge McCormick, who has first dibs on any proceeding that comes before the court, chose herself of among a court of seven judges to oversee one of the most high profile corporate court battles in years.

At a hearing in September, as lawyers for Mr. Musk argued to delay the trial to take into account new claims from a whistle-blower, she poked at the billionaire’s decision to skip due diligence in his race to sign the deal in April. When Mr. Musk’s lawyer argued it would have been impossible to find out about the whistle-blower before the deal, she interjected, “We’ll never know, will we?” She added that “there was no due diligence.”

She made clear in hearings that she did not believe Mr. Musk had handed over all of the messages sent by text and on the app Signal that he was asked to turn over as evidence in the court fight. Twitter’s lawyers revealed a seemingly one-sided text message exchange between Mr. Musk and a banker at the investment bank Perella Weinberg Partners. The banker texted a question and, 18 minutes later, wrote: “OK. Got it.” Mr. Musk’s response was missing.

“Assuming that Musk’s response was not telepathic, one would expect some evidence of it,” Judge McCormick wrote in a ruling.

“She evidently was not putting up with any nonsense,” said Lawrence Hamermesh, a professor of law at Delaware Law School.

In October, after weeks of presiding over bruising back and forth arguments between the two sides, Judge McCormick granted Mr. Musk’s requests to put the trial on hold to give him more time to complete his financing for the acquisition. Judge McCormick granted him until Oct. 28 — a three-week delay.

“She had one eye on the clock,” said Brian Quinn, a professor at Boston College Law School, noting the two sides did not seem ready for a trial just two weeks away. “Another eye,” Mr. Quinn said, was “on potential appeals. She is looking forward saying, ‘Well, what if I ruled against Musk, and he appealed, and his appeal is that I pushed him — I rushed him toward the trial when he wanted to close the deal.”

Judge McCormick is well-versed in trials involving deals with buyers that tried to walk away. As an associate at the law firm Young Conaway Stargatt and Taylor, she worked on cases involving deals that went awry when the stock market crashed in 2008. That included representing the chemical company Huntsman in 2008 when the private equity firm Apollo Global Management scuttled the deal it had struck to combine the chemical company with another it owned.

That deal, and others like it, paved the way for the kinds of contracts Twitter signed with Mr. Musk. Sellers learned how to prevent buyers from trying similar escape hatches. Companies increasingly structure deals with “specific performance” clauses allowing them to force a deal to close.

As chief judge of the Delaware Chancery Court, Judge McCormick presided over a rush of cases involving buyers trying to break off deals they struck just before the pandemic. In one of those cases last year, she forced a buyer, the private equity firm Kohlberg and Company, to follow through with its acquisition of a cake supplier after it argued that the pandemic had materially damaged the business by curbing demand for party cake.

Kohlberg contended it could not complete the deal because its debt financing had fallen apart. Judge McCormick did not buy that argument.

If Mr. Musk does not come through with Twitter’s money by Friday, that could ding his credibility in court, legal experts say. That could matter in November, when Judge McCormick is set to preside over a separate trial involving Mr. Musk and his compensation.

The case, filed in 2018, had originally been assigned to another judge on the Delaware Chancery Court, Joseph R. Slights III, before he retired in January. Judge McCormick picked up the case on Jan. 12, the same month Mr. Musk began to buy up shares of Twitter stock that ultimately led to his planned purchase of the company.

“It’s not ideal for him,” said Ann Lipton, a professor of corporate governance at Tulane Law School, of Mr. Musk’s multiple run-ins with Judge McCormick. “She’s uniquely low drama, which is the opposite of Musk. ”