Baker McKenzie (Illinois state) : The largest law firm in the United States

Co-founding partner Russell Baker, born in Wisconsin and raised in New Mexico, opened his early practice, Baker & Simpson, in Chicago, in 1925, following graduation from the University of Chicago School of Law. Baker had early exposure to the Spanish language and other cultures, and his firm provided legal services to Chicago's growing Mexican American community. The firm later advised U.S. companies investing in Latin America.

In 1949, the firm relaunched with John McKenzie, a litigator who had graduated from Loyola University Chicago School of Law, who took charge of the litigation practice, as Baker built an international practice. Through the 1950s, the firm's client roster expanded.

In 2001, the Firm employed 3000 attorneys and garnered $1 billion in revenues. In 2005, 70 partners, and other legal staff, from the New York office of disbanding international firm Coudert Brothers joined Baker McKenzie.

In August 2014, Baker McKenzie revealed it was the first law firm to break through the $2.5bn revenues barrier since the financial crisis, and that it was the largest firm in the world by headcount.

The firm's global chair, from 2016, until his death in April 2019, was former London managing partner Paul Rawlinson, who was the 15th chair, and the first British Chair of the firm. Rawlinson had succeeded Eduardo C. Leite who was chair of Baker McKenzie from 2010 to 2016

Baker McKenzie became an international firm beginning in 1955, when a lawyer in Venezuela contacted Baker McKenzie about opening a joint venture office in Caracas. Russell Baker's son, Donald, moved to Caracas to launch the satellite office. Within the next three years, offices were opened in Washington, D.C., Amsterdam, Brussels, Zurich, New York and São Paulo.

Baker McKenzie was also the first international law firm to enter many local markets in the Asia Pacific region, starting with the Philippines, in 1963, followed by Sydney, Tokyo, Hong Kong, Bangkok, and Taipei, attaining the largest geographical spread among international law firms within the region, with 17 offices in total.

Rather than practicing U.S. law abroad, Baker McKenzie trained local lawyers, often bringing them to its Chicago base for an initial period, or temporarily relocating U.S. attorneys to the foreign office, to oversee the establishment of the practice. By 1978, Baker McKenzie had 26 offices in 20 countries.

According to wikipedia