In 1807 Millar published with William VazieObservations on the Advantages and Practicability of making Tunnels under Navigable Rivers, particularly applicable to the proposed Tunnel under the Forth, Edinburgh. This work related to a plan for a tunnel from Rosyth to Queensferry, also involving John Grieve. Robert Bald was consulted, but the project came to nothing.
Millar also edited the fourth edition of the Encyclopædia Britannica, 20 vols., Edinburgh, 1810, and the last fifteen volumes of the fifth edition of the same work, 20 vols., Edinburgh, 1817; he contributed largely to both editions. Millar also planned and edited a more popular dictionary of arts, sciences, and literature, the Encyclopædia Edinensis, 6 vols., Edinburgh, 1827.
Other publications, mainly based on articles already contributed to encyclopædias, included: 
A Guide to Botany, Edinburgh, 1819.
Elements of Chemistry, Edinburgh, 1820.
Practical Observations on Cold and Warm Bathing, and descriptive Notices of Watering-places in Britain, Edinburgh, 1821.