In 1815, at the age of 19 years, he left Ireland for Calcutta, India, where he set up as an architect designing private houses for the merchants of Fort William.

[3] Coleman then obtained an introduction to Sir Stamford Raffles from Palmer in Calcutta, and travelled to Singapore, arriving in June 1822.

On his return, Raffles approved the house, construction of which was begun in November of the same year on Singapore Hill and completed in January 1823.

In June 1823, Coleman left for Java where he spent his next two and a half years, and returned to Singapore in 1825 due to conflicts between the Dutch and native Javanese.

[6] In June 1827, Coleman was employed as a Revenue Surveyor, surveyed land titles which was issued mostly to cover shop-house lots in the town.

Coleman helped cofound the establishment of the Singapore Free Press & Mercantile Advertiser newspaper with William Napier, Edward Boustead, and Walter Scott Lorrain.

He also built the first Anglican church in Singapore, St Andrew's, which was begun in 1835, but this structure was demolished in the 1850s having become unsafe due to lightning strikes.

On a return trip to in Ireland, Coleman married Maria Frances Vernon, of Clontarf Castle, Dublin on 17 September 1842.

Coleman however found himself unable to settle down in Europe, and returned to Singapore via Calcutta with his bride on 25 November 1843 at short notice.

Napier adopted Coleman's infant son George, who would die on board of HMS Maeander at sea in 1848 at age 4.