In Baltimore (home to other prominent United Irish exiles, including William Sinclair, Henry Jackson and John Devereux), White practiced medicine (on the corner of East and Holliday Streets), He was naturalised as a U.S. citizen in 1801 (albeit as "John Campbell", a clerical error corrected in 1839); was an officer of the Medical and Chirurgical Faculty of Maryland in 1802; a founder and first president of the Baltimore Benevolent Hibernian Society in 1803; a founder and trustee of Baltimore College in 1803; and a consulting physician for the Baltimore Hospital in 1812 and 1818. During the War of 1812 he played a leading role in the defence of Baltimore against the British, serving as a member of the ‘committee of supply’ which had at its disposal $20,000 for defensive purposes.