I will speak to the legal profession in the U.S., which is what I am familiar with.

To begin with, I want to note that in continental Europe, the law has long been a "universitized" profession. The "civil law" tradition of the Continent has its roots in Roman law, and more specifically the highly intellectualized study of Roman law that began in Italian universities in the 11th century and spread across the Continent. Lawyers on the Continent by and large studied law at a university in a highly academic environment. England, however, was largely disconnected from this intellectual tradition at the time of the Norman Conquest, when it began to develop its distinctive "common law" system. England did not develop a rigorous academic approach to the law; budding legal minds trained at "Inns of Court" under the supervision of actual lawyers rather than at universities. The United States, naturally, inherited this approach, and lawyers generally saw themselves as engaging in a trade or vocation, rather than an intellectual discipline. Until the late 1800s, it was quite rare for lawyers in America to be trained at a university. Most people became lawyers through an apprenticeship, studying under a practicing attorney for several years. Some studied law on their own, perhaps most famously Abraham Lincoln.

Nevertheless, a handful of American universities did have professors of law beginning in the late 18th century, most prominently George Wythe at the College of William and Mary in 1779. The Litchfield School, the first recognizable "law" school in America, was founded in in 1784 and lasted till 1833, and trained about 900 people, many of them prominent politicians. A handful of law schools popped up across the country--31 by the year 1870--but the vast majority of lawyers did not train at law schools, and had no academic contact with the law at all. Prospective lawyers would continue to train under current practitioners and read legal commentaries and treatises. Admission to the bar at this time did not require university attendance--just training under an actual lawyer--so attending classes to learn the law offered little to no benefit for the vast majority of practitioners. These early law schools did pave the way, however, for the formalization of the legal profession that began in earnest in the 1870s.

In 1870, Christopher Columbus Langdell became dean of Harvard Law School, which at the time was a small institution. Langdell hired full-time legal faculty, professionalizing the practice of teaching the law. This was critical to the development of the law as an academic pursuit; before, most law professors were practitioners who occasionally taught a course. At Harvard, they were full-time intellectuals, helping shift the law from a vocation to an intellectual profession.

Langdell also introduced a teaching methodology that every law student today must suffer through--the casebook method. In the casebook method, students learn the law by studying actual legal cases, reading them to understand how judges think about and develop the law. This was clearly quite different from the practical training in law that most lawyers would receive in an apprenticeship. It emphasized law as an intellectual endeavor, not a purely vocational one.

The use of cases to teach law was likely made easier by the creation of the West Reporters in 1879, a series of books that reports and publishes cases by courts in different jurisdictions. The West Reporters made cases far more accessible to the public. In addition, the West Reporters revealed the remarkable disunity of American law from jurisdiction to jurisdiction. The assumption that the law was largely the same across state lines as part of a universal "common law" was shown to be false, perhaps incentivizing more systematic study of the law.

In large part due to the great prestige of Harvard, Langdell's reforms spread gradually across the United States. At the same time, and again probably due to Harvard's great prestige and the desire to imitate it, the number of law students across the country increased dramatically; by 1894 there were 7600 law students in the U.S., compared to just 1611 when Langdell took over Harvard Law.

The development of law reviews, which are highly prestigious student-run academic journals, also reflected the evolution of American law from a trade to an intellectual intellectual course of study. The Harvard Law Review began publication in 1887 (though not the first of its kind, it was certainly the most influential); other law schools created their own too. Law reviews became a place for legal scholars to advance their ideas and further advanced the notion that law was something to be studied, not merely practiced.

Langdell's reforms coincided with a growing professionalization of the legal world. The American Bar Association was founded in 1878 to help advocate for increased regulation of the legal profession, including educational requirements. In 1900, the Association of American Law Schools was formed, and in 1901 issued requirements for membership in its elite circle that included admissions standards and length of study; in 1906, the Association required members to have a 3-year program of study, which is now universal. Through the first half of the 20th century, states gradually adopted law school requirements for bar admission, and law schools adopted more rigorous admissions and graduation standards. It should be noted that it took a long time for such standards to become universal--by 1953, 20% of lawyers had not technically graduated law school!

Sources:

Albert James Harno, Legal Education in the United States: A Report Prepared for the Survey of the Legal Profession (1953)

Courts, Judges, and Politics: An Introduction to the Judicial Process (Walter F. Murphy & C. Hermann Pritchett, eds, 3d. ed. 1979)

John Henry Merryman & Rogelio Perez-Perdomo, The Civil Law Tradition: An Introduction to the Legal Systems of Europe and Latin America (4th ed. 2019)