Cooper was appointed a member of the Executive Committee of the Western New York Anti-Slavery Society in December 1848. When women of Williamson organized an antislavery fair in January 1849, Griffith Cooper introduced their featured speaker, John S. Jacobs, sister of Harriet Jacobs, who later wrote Incidents in the Life of a Slave Girl. Both John S. Jacobs and Harriet Jacobs had escaped from slavery in Edenton, North Carolina. Frederick Douglass was also a speaker at that meeting. In September 1849, Cooper spoke, along with Frederick Douglass, at a meeting in Marion, Wayne County. 

When Abby Kelley Foster and Stephen Foster came through western New York in 1852, Cooper organized an antislavery convention in Williamson on March 22-23 “at the meetinghouse formerly occupied by the Society of Friends.” Wesleyan, Presbyterian, and Baptist churches all held a “protracted meeting,” i.e. a revival, instead of allowing this convention. Debate centered on the contention that “the church is mainly responsible for the continuance of this most atrocious and abhorrent system [of slavery],” and “the best means of accomplishing this work, is the faithful preaching of the truth, and not by the action of political parties.” Although Samuel C. Cuyler, keeper of the next Underground Railroad to the north was a major political abolitionist, the meeting affirmed Cooper’s own idea that “a sectarian Religion will always, in the trying hour, prove untrue to Liberty.”

Griffith Cooper sent the notice of this convention to the National Anti-Slavery Standard with the note that the Wesleyan Methodists had read the notice of the meeting, “as a special favour,” “and then desired, or, in other words, prohibited, ‘all praying souls’ from attending it. Such is the character of all such religion—it abounds in faith, but works are left for those, whom they denounced as Infidels. Thine for the slave. G.M. Cooper.” At the eighteenth anniversary of the American Anti-Slavery Society, held at Corinthian Hall in Rochester in May 1852, and chaired by William Lloyd Garrison, Cooper was elected a manager. 

Griffith Cooper died in Williamson on September 22, 1864, having been in ill health for some time. The Pultneyville Commercial Press printed a memorial passed at the annual meeting of the Friends of Human Progress:

Since our last meeting Griffith M. Cooper, a venerable man, ripe in wisdom and experience, who has been with us, at times in person, always in spirit and desire, has passed on to a higher life.

Through a long and varied career; as a Naval Officer, as a preacher of the Society of Friends, as a guardian of the Indians on the Cattaraugus Reservation from the rapacity of speculators, (employed in the capacity by the society of which he was a member), as an early and faithful advocate of Freedom, Peace and Temperance, as a reformer, stepping beyond the limits of sect, and transcending the bounds of arbitrary authority, he sought to be true to his own light.

His clear, and frank, and faithful word was ready in public or private for the best truth he knew and felt.

We offer this brief testimonial to the genial qualities, and manly virtues of a much-beloved friend, who, his course on earth well finished, has gone to meet those of his dear family "not lost but gone before.” 

Griffith M. Cooper was buried in the Quaker cemetery on the corner of Route 21 and Pearsall Road, across the street from his home. Elizabeth Hodgson Cooper died April 1, 1873, in New Orleans, Louisiana.