Monboddo's main work, Of the Origin and Progress of Language (6 vol., 1773–92), contains a vast body of curious lore on the manners and customs of primitive peoples, relates man to the orangutan, and traces his development to a social state. Some of Monboddo's ideas and habits earned him a reputation as an eccentric: he believed, for instance, that children are born with tails, and at his dinner parties the table was strewn with roses in emulation of the Roman poet Horace. His sayings, whims, and oddities became legendary in his lifetime.

He assumed the title Lord Monboddo in 1767 when he became a judge as ordinary lord of session, in Edinburgh.