Born and raised in Plainfield, New Jersey, Cary earned a Bachelor of Arts degree in 1949 from Morehouse College, where he served as student body president.

[3][4] In 1944, the Courier News called Cary "the boy preacher" in an announcement of his sermon for Young People's Day at a local AME church.

[1][11] Cary continued his studies at Union Theological Seminary in New York City while also serving as a student assistant to the minister of Grace Congregational Church.

[11][14] During that time, he co-chaired the United Negro College Fund in Youngstown and was active in other local organizations and committees including the YMCA and the Mahoning County Mental Health Council.

[12][20] He changed his denominational affiliation from Baptist to the United Church of Christ (UCC)[21] and began his new position on September 1, 1958, succeeding Herbert King who had resigned to become a professor at McCormick Theological Seminary.

[27] At the annual UCC assembly in 1966, he condemned the Ku Klux Klan and other "insane bigoted mobs", and forcefully called for high-quality integrated schools and fairer employment laws.

"[31] After his lectures, the conference ministers voted to adopt several race-related resolutions, including lobbying the state government for open housing and encouraging churches to appoint more Black pastors.

[37] The Minneapolis Star reported in September 1972 that Cary would be put forth by a nominating committee for president of the National Council of Churches, the largest ecumenical body in the United States, and that he was expected to be elected.

[40][14] At the Dallas meeting, the NCC general assembly also voted to establish a new, more diverse governing body to be rolled out during Cary's first year in office.

[43] In February 1973, Cary joined other religious leaders, including Paul Moore Jr. and Balfour Brickner, in criticizing President Richard Nixon's proposed budget which decreased funding for affordable housing and other anti-poverty measures.

[45] After Nixon fired Archibald Cox during the Watergate scandal, Cary released a statement urging Congress to "examine the President's fitness to remain in office".

[48] That same month, Cary and NCC General Secretary R. H. Edwin Espy apologized for and retracted a statement they had sent to the House Committee on Ways and Means opposing tax credits for students attending private schools, after they had received backlash from Catholic bishops.

[56] The meeting re-established the relationship between the NCC and the White House, though Ford and church leaders continued to disagree on issues like amnesty and Vietnam aid.

[58] In March 1975, the NCC voted for the first time to support gay rights passing a resolution that condemned discrimination on the basis of "affectional or sexual preference".

[59] At the same meeting, the governing board voted in support of the Equal Rights Amendment and resolved to investigate Cary's claim that the Nixon administration had bugged NCC phones in order to conduct special tax audits of the organization.

[63] Cary's three-year term as NCC president ended on October 11, 1975, when the governing board elected William Phelps Thompson who was the chief executive of the United Presbyterian Church at the time.