 He was the son of Ann (née Dillon) and Patrick "Patt" Lalor, a landowner and supporter of the abolition of tithes who was a member of the British parliament (MP) in 1832–1835; Patt Lalor was the first Catholic MP from Queen's County since the anti-Catholic Test Acts of the 17th century.He had 11 children, of whom Peter was the youngest.The eldest brother was James Fintan Lalor, who was later involved in the Young Ireland movement and the unsuccessful rebellion of 1848.