Williams first joined the NAACP, but later became a leader in the SCLC along with Martin Luther King, Jr., Ralph Abernathy, James Bevel, Joseph Lowery, and Andrew Young among many others. He played an important role in the demonstrations in St. Augustine, Florida that some claim led to the passage of the landmark Civil Rights Act of 1964. While organizing during the 1965 Selma Voting Rights Movement he also led the first attempt at a 1965 march from Selma to Montgomery, and was tear gassed and beaten severely. The Selma demonstrations and his "Bloody Sunday" attempt led to the other great legislative accomplishment of the movement, the Voting Rights Act of 1965. After leaving SCLC, Williams played an active role in supporting strikes in the Atlanta, Georgia area by black workers who had first been hired because of the Civil Rights Act of 1964.

In the 1966 gubernatorial race, Williams opposed both the Democratic nominee, segregationist Lester Maddox, and the Republican choice, U.S. Representative Howard Callaway. He challenged Callaway on myriad issues relating to civil rights, minimum wage, federal aid to education, urban renewal, and indigent medical care. Williams claimed that Callaway had purchased the endorsement of the Atlanta Journal. Ultimately, after a general election deadlock, Maddox was elected governor by the state legislature.

In 1972, Williams ran in the Democratic primary for the U.S. Senate seat formerly held by the late Richard Russell, Jr. He polled 46,153 votes (6.4 percent). The nomination and the election went to fellow Democrat Sam Nunn. In 1974, Williams was elected to the Georgia Senate where he served five terms as a Democrat, until 1984. In 1985 he was elected to the Atlanta City Council, serving for five years, until his election in 1989 when he ran for Mayor of Atlanta but lost to Maynard Jackson. That same year Williams successfully campaigned for a seat on the DeKalb County, Georgia County Commission which he held until 1994. Williams supported former Georgia Governor Jimmy Carter for President in 1976, but surprised many black civil rights figures in 1980 by joining Ralph Abernathy and Charles Evers in endorsing Ronald Reagan. By 1984, however, he had soured on Reagan's policies, and returned to the Democrats to support Walter F. Mondale.

On January 17, 1987, Williams led a "March Against Fear and Intimidation" in Forsyth County, Georgia, which at the time (before becoming a major exurb of northern metro Atlanta) had no non-white residents. The ninety marchers were assaulted with stones and other objects by several hundred counter-demonstrators led by the Nationalist Movement and Ku Klux Klan. The following week 20,000, including senior civil rights Leaders and government officials marched. Forsyth County began to slowly integrate in the following years with the expansion of the Atlanta suburbs. In 1971, Hosea Williams founded a non-profit foundation, Hosea Feed the Hungry and Homeless, widely known in Atlanta for providing hot meals, haircuts, clothing, and other services for the needy on Thanksgiving, Christmas, Martin Luther King, Jr. Day and Easter Sunday each year. Williams' daughter Elizabeth Omilami serves as head of the foundation. In 1974, Williams organized the International Wrestling League (IWL), based in Atlanta, with Thunderbolt Patterson serving as President. Among other entrepreneurial endeavors, he founded Hosea Williams Bail Bonds, Inc., a bail bond agency.

In the early 1951, Williams married Juanita Terry (1925–2000). Together, Williams and Terry had eight children. Four sons: Hosea L. Williams, II (1955–1998), Andre Williams, Torrey Williams, and Hyron Williams, and four daughters: Barbara Williams–Emerson, Elizabeth Williams–Omilami, Yolanda Williams–Favors, and Jaunita Williams–Collier. Williams died at Piedmont Hospital in Atlanta, after a three-year battle with cancer on November 16, 2000. Funeral services were held at the historic Ebenezer Baptist Church, where close friend Martin Luther King Jr, was once the co-pastor. Williams was preceded in death by his wife three months prior and by his son Hosea II two years earlier. Williams is interred at Lincoln Cemetery.

Boulevard Drive in the southeastern area of Atlanta was renamed Hosea L Williams Drive shortly before Williams died. Hosea Williams Drive runs by the site of his former home in the East Lake neighborhood at the intersection of Hosea L. Williams Drive and East Lake Drive. Hosea L. Williams Papers are housed at Auburn Avenue Research Library On African American Culture and History in Atlanta. His daughter Elisabeth Omilami also maintains a traveling exhibit of valuable civil rights memorabilia. Williams was portrayed by Wendell Pierce in the 2014 film Selma.