[1] After his father's death while he was about sixteen, his uncle, John Foster, oversaw his further education, encouraged him to travel and employed him (presumably part-time) as his private secretary (in an office for the loss of which he was later compensated on the Union with Great Britain with an annuity of £10 5s).
His travels continued later that year when he set out in July on a tour of Europe encompassing Switzerland, Italy, Austria, Prussia, the Black Sea and Constantinople[3] before returning to Dublin in September 1803.
He returned to the bar in 1812, but in 1816 was brought back to Parliament at the instigation of the government as member for Sir Leonard Holmes's borough of Yarmouth on the Isle of Wight.
On 24 June 1824, he was appointed to the Royal Commission for inquiring into the nature and extent of the Instruction afforded by the several Institutions in Ireland established for the purpose of Education where he served with the other Commissioners: Thomas Frankland Lewis, William Grant, James Glassford and Anthony Richard Blake.
[8] At the Co. Louth Election in August 1826 John Leslie Foster was knocked down to second place in the two-seat constituency by Alexander Dawson, a candidate put up by O’Connell's ascendant Catholic Association.
Although John Leslie Foster was (as he assured the House of Commons in February 1829) ‘no Orangeman’, he was a persistent opponent to Catholic Emancipation.
Lord Ellenborough recorded that ‘Peel told us he had seen [John] Leslie Foster who was for a settlement, but strongly against paying the Roman Catholic clergy.