He worked in Manchester during 1816 before returning home to Ireland, where he attended Samuel Nielson's school in Dundalk, and with his younger brother John (b.

In 1827, he designed the Godmanchester Chinese Bridge which crosses a mill stream of the River Great Ouse in 1827, and then worked on the redevelopment of the Grosvenor Estate in Mayfair, along with commissions for college buildings, prisons, and factories.

[3][5][1] Arriving in the US in New York, Gallier formed a brief partnership with his exact contemporary Minard Lafever (1798–1854), and published The American Builder's General Price Book and Estimator (1833).

[6] From Lafever undoubtedly Gallier met the brothers Charles (1811–1839) and James Dakin (1806–1852), who were then working for the prominent New York architects Ithiel Town and Alexander Jackson Davis.

[7] Town and Davis, and James Dakin on his own in 1834, were in the midst of designing some of the most distinguished Greek-revival buildings in the United States at the time, including the Bank of Louisville in Kentucky (1834), as well as the First Presbyterian Church in Troy, New York (1835).

Amongst the earliest and most significant was the St. Charles Hotel (1835–37), which was one of the first large buildings constructed on Canal Street, which would develop into the city's main commercial artery.

This impressive Greek revival structure had a 6-column projecting Corinthian portico, marble front steps, and huge dome, second only in size in the US to the cupola of the United States Capitol in Washington, D.C.

At the top of the spiral stairs at the base of the dome was a large 11-foot (3.4 m) wide gallery, which afforded views of the entire city at a height of 185 ft (56 m).

In 1838 Dakin designed St. Patrick's Church, an ambitious effort in a rich Gothic style, supposedly modeled on York Minster.

[13] Charles also established a branch of the firm in Mobile, Alabama, and there the Dakins and Gallier completed Barton Academy in 1836 and Government Street Presbyterian Church (1836), now a National Historic Landmark.

A large fire in Mobile in 1839 destroyed much of the firm's unfinished work there; that same year Charles apparently became depressed after the collapse of a row of warehouses he designed and left for Texas to start over, but died later in 1839 in St. Gabriel, Louisiana.

[9] In this now-central part of New Orleans, which stretches from Canal Street to the Garden District, Gallier was commissioned to build a new structure housing many municipal services for this district, one of three "municipalities" that the city had been effectively divided into in an effort to reduce tensions between the different ethnic groups of Creoles, Anglophones, French, Spanish, and Free People of Color, an arrangement that soon proved unworkable.

[17] Another complex surely seen by nearly every visitor to New Orleans are the group of two nearly identical sets of townhouses (now apartments and commercial ground-level shops) flanking the north and south sides of Jackson Square in the French Quarter or Vieux Carré, built in 1849–51 and known as the Pontalba Buildings.

On 3 October 1866, James and Catherine Gallier were passengers on board the Evening Star, a paddle-wheel steamer en route from New York City to New Orleans, when it sank in a hurricane about 175 miles east of Savannah, Georgia.