On August 1st, 1863, Friedrich Bayer and Johann Friedrich Weskott founded a general partnership called 'Friedr Bayer et comp' in Barmen, Germany. They aimed to manufacture and sell synthetic dyestuffs, which was an innovative approach in the textile market at that time.

In 1866, the headquarters and most production facilities of the company moved from Barmen to a larger area in Elberfeld, marking a significant expansion.

Friedrich Bayer, the son of the company's founder, joined the company as a chemist in 1873, contributing to its growth and development.

After the death of Friedrich Bayer's father in 1880, the company transitioned into a joint-stock company known as Farbenfabriken vorm. Friedr. Bayer & Co, also called Elberfelder Farbenfabriken.

In 1881, Bayer was transformed into a joint stock company called 'Farbenfabriken vorm. Friedr. Bayer & Co'. The workforce grew significantly from three staff in 1863 to over 300 by 1881.

In 1895, Bayer developed Heroin as an over-the-counter drug for pain relief. This was one of the pharmaceutical products that emerged from Bayer's research capabilities.

In 1899, Bayer invented Aspirin, which was developed by Felix Hoffman and became known as the 'drug of the century'. This marked a significant milestone in Bayer's pharmaceutical innovations.

In 1903, Bayer licensed the patent for the hypnotic drug diethylbarbituric acid from its inventors Emil Fischer and Joseph von Mering. This led to the discovery of phenobarbital in 1911, known for its potent anti-epileptic activity.

In 1904, Bayer introduced its iconic corporate logo, the Bayer cross, featuring the word BAYER written vertically and horizontally, enclosed in a circle, which became a symbol of the company.

Bayer relocated its headquarters from Barmen to Leverkusen, north of Cologne, in 1912. This move marked a significant development in the company's history and solidified its presence in the chemical and pharmaceutical sectors.

In 1913, Bayer expanded its presence by creating additional foreign affiliated companies in countries like the United Kingdom, United States, Russia, Belgium, and France. This expansion aimed to secure and strengthen Bayer's position in international markets.

In 1916, Bayer scientists discovered suramin, an anti-parasite drug still sold under the brand name Germanin. The formula was elucidated and published in 1924.

In 1924, the U.S. Congress prohibited the sale of heroin due to its harmful effects and addictive nature.

In 1925, Bayer merged with five other German companies to form IG Farben, creating the world's largest chemical and pharmaceutical company. This merger marked a significant development in Bayer's history.

By 1926, IG Farben had amassed three times more assets than all other chemical companies in Germany combined, solidifying its position as a powerful conglomerate in the industry.

Bayer scientist Gerhard Domagk received the 1939 Nobel Prize for his discovery of the antibacterial drug Prontosil, a significant milestone in the field of pharmacology.

During World War II, IG Farben, Bayer's parent company, used slave labour in factories built in Nazi concentration camps, notably in Auschwitz III. Almost half of IG Farben's workforce consisted of slave labour or conscripts by 1943.

After World War II, some employees of Bayer were involved in the IG Farben Trial under US jurisdiction. Fritz ter Meer, who helped plan the Monowitz camp and IG Farben's Buna Werke factory at Auschwitz, was sentenced to seven years but released in 1950. He later served on Bayer AG's supervisory board.

In 1951, the Allied Control Council seized IG Farben for its involvement in building and maintaining German war potential. The company was split into its six constituent companies, which were further divided into three, including BASF, Bayer, and Hoechst.

In 1953, Bayer introduced the first neuroleptic, chlorpromazine, to the German market.

In 1956, Fritz ter Meer, who was convicted of war crimes for his actions at Auschwitz, was elected to Bayer AG's supervisory board. He held this position until 1964.

In 1974, Bayer acquired Cutter Laboratories Inc., which helped the company gain a significant position in the US pharmaceuticals market.

In 1977, Bayer was compelled by a United States antitrust suit to purchase Monsanto's share of Mobay, resulting in $540 million in sales.

In 1978, Bayer acquired Miles Laboratories Inc., further strengthening its presence in the US pharmaceuticals market.

In 1982, Bayer underwent a restructuring process that involved the creation of a third tier below the management board.

In 1983, the CDC identified contaminated blood products as a source of infection during the early days of the AIDS epidemic, leading to higher rates of AIDS among people with hemophilia. Bayer and other companies developed new methods to decontaminate blood products.

In 1984, Hermann Josef Strenger took over as Management Board Chairman of Bayer, leading the company through a period of growth and acquisitions.

In 1986, Bayer acquired Hermann C. Starck GmbH, a key supplier of specialty metals and high-performance ceramics, expanding its portfolio.

In 1988, Bayer marked its 125th anniversary with sales reaching approximately DM 40 billion and a global workforce of over 165,000 employees.

Up to 650 surgeries were performed using Ciproxin without doctors being informed about its adverse reactions with other drugs, leading to impaired bacteria-killing ability.

In the 1990s, the federal government initiated an investigation into Bayer's billing practices.

In 1992, Bayer unified its United States operations under the name Miles, discontinuing the use of the Mobay name.

In 1993, Miriam Kor, sister of Eva Mozes Kor, passed away due to complications from the experiments she was subjected to as part of Bayer's experiments during World War II.

In 1994, Bayer regained the rights to its company name in the United States after 75 years. This allowed the company to operate in the US under its own name and logo, the Bayer Cross, under the leadership of Dr. Manfred Schneider.

In 1995, Helge Wehmeier, then CEO of Bayer, publicly apologized to Elie Wiesel for the company's actions during World War II and the Holocaust.

In 1997, Bayer and three other companies agreed to pay $660 million to settle cases on behalf of over 6,000 hemophiliacs infected in the United States due to contaminated blood products.

On March 6th, 1999, Bayer marked the 100th birthday of Aspirin by wrapping its former high-rise headquarters building in Leverkusen to create the world's largest Aspirin pack. This achievement earned Bayer three entries in the Guinness Book of Records.

Bayer resolved the lawsuit regarding its involvement in human experimentation by reaching a settlement with the Foundation for Remembrance, Responsibility and the Future. The settlement was part of the $5 billion fund negotiated by the U.S. and German governments.

In 2001, Bayer introduced the women's contraceptive pill Yasmin to the market. Marketed as a treatment for acne, severe premenstrual symptoms, and birth control, Yasmin became a significant product in Bayer's pharmaceutical portfolio.

In 2003, documents revealed that Cutter, acquired by Bayer, continued to sell unheated blood products in markets outside the US until 1985, potentially exposing more individuals to infections. Bayer cited doubts about the efficacy of new products in some countries as a reason for this.

In 2005, the US FDA approved Nexavar (sorafenib), a drug developed by Bayer HealthCare and Onyx Pharmaceuticals for the treatment of advanced renal cell carcinoma.

On December 15, 2006, the FDA restricted the use of Trasylol due to findings of greater risks of death, serious kidney damage, congestive heart failure, and strokes associated with the drug.

In November 2007, the FDA requested that Bayer suspend the marketing of Trasylol, further emphasizing the concerns over its safety and potential risks.

An explosion occurred at the Bayer CropScience facility in West Virginia, resulting in the death of two employees. The incident involved a runaway reaction that ruptured a tank.

2010 marked fifty years since the regulatory approval of the contraceptive pill in 1960, with Bayer maintaining a leading position in hormonal contraception.

In 2011, Bayer received FDA approval for Eylea, an eye medicine used in the treatment of wet age-related macular degeneration.

In 2013, the European Medicines Agency lifted the suspension of Trasylol marketing authorization for selected patients undergoing cardiac bypass surgery, based on a favorable risk-benefit ratio.

In May 2014, Bayer announced the acquisition of Merck & Co's consumer health care unit for $14.2 billion, expanding its portfolio of healthcare products.

In June 2015, Bayer sold its Diabetes Care business unit to Panasonic Healthcare Co. for $1.15 billion, reshaping its medical devices portfolio.

In September 2015, Bayer spun out its $12.3 billion materials science division into a separate, publicly traded company called Covestro, retaining about a 70% interest.

In May 2016, Bayer offered to buy U.S. seeds company Monsanto for $62 billion, leading to a $66 billion agreement in September 2016 after regulatory approvals.

In fiscal year 2017, Bayer reported earnings of EUR€7.3 billion with an annual revenue of EUR€35 billion, showing a decrease of 25.1% compared to the previous fiscal cycle.

Bayer settled over 8,000 Yaz lawsuits filed by more than 10,000 women for $1.7 billion due to reported side effects associated with the hormonal birth control.

In August 2019, the business acquired approximately 60% of BlueRock Therapeutics for up to $600 million.

On February 14, 2020, Bayer and BASF were instructed to compensate Missouri peach farmer Bill Bader with $15 million for the destruction of his peach trees due to dicamba usage by neighboring farmers. The dicamba product was acquired by Bayer from Monsanto, inheriting the lawsuit as well.

On February 15, 2020, Bayer (representing Monsanto) and BASF were directed to pay an extra $250 million in punitive damages on top of the initial $15 million to Missouri peach farmer Bill Bader. Following this, Bayer and BASF announced their intention to appeal the total $265 million fine.

On November 25, 2020, U.S. District Judge Fernando M. Olguin turned down Bayer's $650 million settlement offer for PCB pollution lawsuits, allowing the related cases involving Monsanto to proceed. This decision impacted the legal resolution of the PCB contamination claims.

In June 2021, the company announced the acquisition of Noria Therapeutics Inc. and PSMA Therapeutics Inc., obtaining rights to cancer-based investigational compounds based on actinium-225.

On 17 June 2022, the US Court of Appeals for the Ninth Circuit ordered the EPA to reexamine the finding that glyphosate in Roundup did not pose a health risk to people exposed to it.

A lawsuit by Merck & Co seeking to hold Bayer AG accountable for additional talc-related liabilities from the 2014 acquisition of Merck's consumer care business was dismissed by a Delaware judge on April 4, 2023. The judge favored Bayer's interpretation of the purchase agreement, protecting the company from further liabilities.