He collaborated with botanists from around the world and served as a regius professor of botany at the University of Glasgow.

He wrote articles in Tilloch's Philosophical Magazine on Observations on the Solution of Exponential Equations (1817) and Comparison between the Chords of Arcs employed by Ptolemy and those now in use (1818).

Finding law boring, he preferred the lectures of Professor Robert Jameson and John Stewart.

He then began to work with Sir William Jackson Hooker, examining the collections from Captain Beechey's voyage and those of other collectors including Alexander Collie.

With his contemporary from high school Robert Wight, he collaborated on plants from India.

He replaced Hooker as lecturer at Glasgow University in 1839 and became Regius Professor of Botany in 1845, a position he held until his death.

In 1839, along with Houston Rigg Brown, Walker Arnott "resuscitated" the Royal Order of Scotland of Freemasonry, which may have been founded before 1732, but by 1819 was on the verge of extinction.