Pierre-Louis Pierson (13 December 1822, Hinckange, Moselle – 22 March 1913, Paris) was a prominent French portrait photographer and co-founder of the esteemed studio Mayer & Pierson, which was one of the foremost photo ateliers in continental Europe during its operation in Paris from 1855 to 1878. In a landmark legal case in 1862, the French Supreme Court recognized "photography" as a distinct visual art form eligible for copyright protection, a significant achievement for Pierson and his contemporaries. Among his notable works is the Scherzo di Follia ("Joke of Madness"), created between 1863 and 1866, stemming from a prolific collaboration with the Italian aristocrat and socialite, Virginia Oldoini, Countess of Castiglione (1837–1899). Their partnership began in 1856, while the Countess was the mistress of Emperor Napoléon Bonaparte III (1808–1873), and lasted until 1895, resulting in over 450 portraits. Pierson was known for directing elaborate "mise-en-scènes" and creating archetypal female characters for the Countess, employing innovative techniques with mirrors and framing. Many of their photographs featured bold poses and revealing attire, which pushed the boundaries of the era’s social norms. Although their intimate collaboration remained somewhat discreet, select photographs were publicly exhibited, including her "Queen of Hearts" portrait at the 1867 Exposition Universelle in Paris.

Pierson began his photography career at the dawn of mass-produced photography in France, opening his daguerreotype portrait studio at №3 boulevard des Capucines in 1844. He quickly identified a lucrative niche in hand-colored prints and was among the first photographers in Paris to adopt the popular carte de visite format, which consisted of small, pocket-sized prints mounted on decorative cardboard. In 1855, Pierson entered into a fruitful partnership with brothers Léopold Ernest (1817–ca.1865/95) and Louis-Frédéric Mayer (1822–1912/13), who had recently been appointed as official photographers to the Emperor. Together, they launched the Mayer & Pierson brand, which featured the imperial coat of arms and the title "Photographes de S.M.L'Empereur." Variants of the brand name included Maison MAYER & PIERSON and Mayer Frères et Pierson.

Over the next decade, Mayer & Pierson achieved remarkable commercial and critical success, establishing itself as the leading photoatelier across Europe. In addition to serving the Second French Empire, Pierson also worked with several other royal families. After Louis Frédéric retired from the partnership in 1861, Pierson and Léopold Mayer continued their collaboration for at least four additional years, during which they opened a studio in Brussels and garnered multiple awards at the 1862 Great London Exposition.

In January 1862, Mayer & Pierson initiated legal action against a competing Parisian studio, Thiebault & Betbéder, for unauthorized reproductions of their carte-de-visites featuring Italian Prime Minister Count Camillo di Cavour. This legal battle, significant in the early history of photography, escalated to the Cour de Cassation, France’s supreme court. The defense argued that photography was merely a "mechanical" reproduction and not a form of artistic expression, thus unworthy of copyright protection. In response, Mayer & Pierson presented a "mini-exhibition" to the judges, showcasing a series of portraits alongside famous paintings to demonstrate photography's artistic value. Ultimately, the Supreme Court ruled in favor of Pierson, affirming photography as a "work of visual art" under French copyright law.

In 1865, after Léopold Mayer's retirement, Pierson became the sole owner of Mayer & Pierson. Thirteen years later, around 1878, he closed the studio and joined the struggling business of his son-in-law, Paul Gaston Braun (1845–1928), a descendant of renowned photographer Adolphe Braun (1812–1877), and his brother-in-law, Léon Clément. Pierson played a crucial role in revitalizing the business, which secured a significant 30-year contract with the Louvre Museum in 1883 to document over 7,000 works in its collection. By 1889, the partners had incorporated as Braun, Clément & Cie; Pierson was 67 at that time, and the precise date of his retirement remains uncertain. He passed away nearly 25 years later at the age of 91, leaving behind a significant legacy in the world of photography.