After graduating from Trinity College in his native city of Dublin in 1830, Mallet joined his father's foundry business. He remained there until 1861 when, with the completion of the Irish railroad network, there was such a marked decline in his business that he moved to London as a consultant engineer. Mallet was an engineer of some skill; he constructed several bridges, the Fastnet Lighthouse, and erected the 133-ton roof of St. George's in Dublin.

As a geologist he is mainly remembered for pioneering observational and experimental seismology. In 1857 he spent two months in the Basilicata region (southeast of Naples) studying the effects of the recent earthquake. He showed that by noting the direction of cracks in walls and the arrangement of fallen masonry, it was possible to determine the epicenter and the depth of the earthquake. His results were published in The Great Neapolitan Earthquake of 1857: the First Principles of Observational Cosmology (1862).

In a series of experiments starting in 1850, Mallet attempted to determine the speed of earthquake waves. He did this by setting off small explosives at different depths in different soils and measuring the time taken for the seismic waves to travel varying distances. James Michel, working in the 18th century, had estimated a speed of 20 miles (32 km) per minute but this could not compare with Mallet's precise measurements.