Prior to his arrival in Australia, Sealy had studied at Trinity College, Dublin. In Sydney, he briefly attended the University of Sydney, where he gained a scholarship, and was among the first group of students to be admitted for study in October 1852. However, he did not complete a degree, and subsequently found employment as a clerk the New South Wales civil service.
Sealy's date of birth remains to be established. The New South Wales Registry of Births, Deaths and Marriages (which has his surname as 'Sealey'), records that he was 29 years old at the time of his death (suggesting a date of birth of ca. 1832-1833). However, University of Sydney records indicate that he was 21 years old when he enrolled there in 1852 (suggesting a date of birth of ca. 1830-1831).