He established the Scotsman, 26 January 1817, with William Ritchie and John M'Diarmid, and was joint editor of the first few numbers. When he obtained a position as a clerk in the custom house, he yielded the editorial chair to John Ramsay M'Culloch In 1820, Maclaren resumed the editorship and held it till 1846, when he resigned it to Alexander Russel. The paper rapidly became the leading political journal of Scotland; its tone was throughout decidedly whiggish, and in church matters it advocated much freedom of opinion. In 1820, Archibald Constable employed Maclaren to edit the sixth edition of the Encyclopædia Britannica, 1823, and to revise the historical and geographical articles. The editor contributed the articles 'America,' 'Europe,' 'Greece,' 'Physical Geography,' and 'Troy,' Maclaren interested himself in science and especially in geology. He was elected F.R.S. Edinburgh in 1837, F.G.S. London in 1846, and was president of the Edinburgh Geological Society from 1864 to his death.
He died at Moreland Cottage, Edinburgh, 10 September 1866. He was interred at the Grange Cemetery.