"[1][2] Stillman was an original trustee of Rhode Island College (now Brown University) and played a leading role in the establishment of the Massachusetts Baptist Missionary Society in 1802.

On a visit to his native Philadelphia, Stillman married Hannah Morgan—the daughter of a Welsh merchant family and sister of noted physician John Morgan.

In 1773, Stillman purchased a house at the northern corner of Sheaffe and Salem Streets in Boston's North End.

According to editor Frank Moore, Stillman was "a member of the Senate Convention for the formation of the state constitution in 1779; as also for the adoption of the federal constitution in 1788; in the last body he delivered a very eloquent speech in its support, and was considered at the time as having contributed much toward its adoption, and confirmed many members in its favor who were previously wavering upon that question.

To that constitution he ever after continued a firm, unshaken friend, and a warm approver of the administration of Washington and Adams.