During Runciman's stay in Italy he became acquainted with other artists such as James Barry, Henry Fuseli and the sculptor Johan Tobias Sergel.

[2] Runciman's earliest efforts had been in landscape; he now turned to historical and imaginative subjects, exhibiting his Nausicaa at Play with her Maidens in 1767 at the Free Society of British Artists, Edinburgh.

He was patronised by Sir James Clerk, decorating the hall of his Penicuik House with a series of subjects from Ossian which took inspiration from Gavin Hamilton's Iliad pictures,[3] and an adjacent staircase with four scenes from the life of Saint Margaret.

[2] He also created various religious paintings and an altar-piece in the Cowgate Episcopal Church, Edinburgh, and easel pictures of Cymon and Iphigenia, Sigismunda Weeping over the Heart of Tancre, and Agrippina with the Ashes of Germanicus.

The grave is unmarked but a stone plaque was erected by the RSA in 1866 on the west-facing wall of the church to his memory (also commemorating his brother John who died in Naples).