An Edinburgh born architect, he trained there under Henry Playfair, who’s celebrated Classicism he later rejected in favour of the Gothic, Romanesque and Italian Renaissance styles.
Setting up his own practice in 1831, he later formed a brief partnership with the Glasgow based civil engineer, William Gale (d. 1858), as Cousin & Gale, 1839-45, during which time he became Edinburgh's Superintendent of Public Works, 1845-72, and architect to the British Linen Bank, 1845-74. He later combined these roles with his appointment as architect to Edinburgh's City Improvement Trust, in 1867.
His work is mainly in Edinburgh, where his finest buildings include:
St. Thomas’s Episcopal Church (1842-3); the Corn Exchange (1853); Reid School of Music (1858); facades in Chambers Street (1864); the Savings Bank and Free Church Building on the Mound (1853-8); and the commercial India Buildings (1862).
For Glasgow, he designed Cambuslang Old Parish Church (1841-3), and the colossal, Romanesque Monteith Mausoleum, in the Necropolis (1842).
The Monteith Mausoleum, as well as being the largest structure in the Necropolis, features forty eight corbel heads under its eaves, each one an individual grotesque carved by an unidentified sculptor.
Cousin also specialized in cemetery design, and was responsible for planning Edinburgh's Warriston Cemetery (1842).
Amongst his domestic buildings is 7 Greenhill Gardens, which he designed for himself (1849).
He spent his later years travelling in search of better health, visiting Algiers, Mentone, and Baton Rouge, Louisiana, where he died on 14th August 1878. His practice was continued by his brother George, who later formed the partnership, Cousin, Ormiston & Taylor, 1905-22.