Bartlett became active in the political affairs of Kingston, and in 1765 he was elected to the colonial assembly. In 1767 he became the colonel of his county's militia and Governor John Wentworth appointed him justice of the peace. As the Revolution neared, his Whig policies brought him into opposition with the Royal Governor, John Wentworth. Although his lodge is not known, his great grandson, Levi S. Bartlett, had a letter written by Josiah to his son Ezra saying, "I attended a Mason meeting last night, and as soon as you can I wish you would join the Masons." In 1774, Bartlett joined the Assembly's Committee of Correspondence and began his work with the revolutionary Leaders of the other 12 colonies. Later that year, when Wentworth dismissed, or prorogued, the Assembly, Josiah was elected to its revolutionary (and illegal) successor, the Provincial Assembly. He also suffered the loss of his home by fire, alleged to have been set by opposition Tories. He moved his family out to the farmhouse and began rebuilding immediately. When the assembly appointed Bartlett and John Pickering as delegates to the Continental Congress, he declined because he wished to attend to his family, but remained active in New Hampshire's affairs. In one of Governor Wentworth's last acts before being expelled from New Hampshire in 1775, he revoked Bartlett's commissions as Justice, Militia Colonel, and Assemblyman.