Macmillan allegedly completed construction of a pedal driven bicycle of wood in 1839 that included iron-rimmed wooden wheels, a steerable wheel in the front and a larger wheel in the rear which was connected to pedals via connecting rods.
A Glasgow newspaper reported in 1842 an accident in which an anonymous "gentleman from Dumfries-shire... bestride a velocipede... of ingenious design" knocked over a pedestrian in the Gorbals and was fined five British shillings. Johnston identified Macmillan as that gentleman.
However, it is said that Gavin Dalzell of Lesmahagow copied the Macmillan machine in 1846 and passed on the details to so many people that for more than 50 years he was generally regarded as the Inventor of the bicycle.
Some historians who have studied the invention of the pedal-driven bicycle, including David V. Herlihy, state that Macmillan was not the first Inventor. Herlihy states there is no contemporary documentary evidence that a pedal-crank design was applied to a 2-wheeled vehicle and that letters from customers in Scotland to the Michaux company in 1868 state that all of the human-powered vehicles there are tricycles and quadracycles. A similar claim is made by David Gordon Wilson.
Johnston did not present conclusive proofs, though he wrote that he had them. Sceptics allege that MacMillan design which he presented was a composite of two 1869 velocipedes by Thomas McCall. At the behest of Johnston, Thomas McCall built a replica to be presented as MacMillan's at the 1896 Stanley show (and now at Dumfries Museum) perhaps for financial reasons.
According to the research of his relative James Johnston in the 1890s, Macmillan was the first to invent the pedal-driven bicycle. Johnston, a corn trader and tricyclist, had the firm aim, in his own words "to prove that to my native country of Dumfries belongs the honour of being the birthplace of the invention of the bicycle".
A 1939 plaque on the family smithy in Courthill reads "He builded better than he knew." Yet MacMillan lived in Glasgow and worked at the Vulcan Foundry during the relevant period around 1840, not in Courthill.