Young James studied at Marischal College and the University of Aberdeen and came to Lower Canada in 1804, where he entered into business.

A good businessman, he achieved rapid success and became a leading member of the Montreal business community.

[1][2] Leslie entered politics in 1824, elected to represent Montreal East in the Legislative Assembly of Lower Canada.

They sought greater popular control of the provincial government, which was dominated by the governor, appointed by the British government, and by the Legislative Council, which was appointed by the governor and largely drawn from the British commercial elite of the province.

Leslie's support of the Parti canadien earned him some hostility from the Montreal business community, which was dominated by British Canadians.

He nonetheless voted in favour of the Ninety-Two Resolutions, which Louis-Joseph Papineau introduced in the Legislative Assembly.

From that point on, rebellion was in the air in Lower Canada, finally breaking out in November 1837, led by Papineau.

Leslie did not support the Lower Canada Rebellion, and was one of the more moderate group of public figures, led by Louis-Hippolyte LaFontaine, who approached Governor General Lord Gosford and asked that the Parliament be recalled.

He stood as a candidate in Montreal County in the first general elections in 1841, but there was extreme electoral violence, which was not uncommon at that time.

A group of French-Canadians seized the poll, and two supporters of the union, one Irish, were battered to death.

However, in late 1841, the member for the Verchères riding, south of Montreal, resigned his seat, allowing Leslie to be a candidate in the subsequent by-election.

[1][2] The Quebec Conference of 1864 was the major meeting of leaders from British North America leading to Confederation.

As part of their framework for the new country, they provided that the upper house of the new Parliament, the Senate of Canada, would be formed by appointment, not election.

The first group of senators would be drawn from the Legislative Councils of the Province of Canada, Nova Scotia, and New Brunswick.

In 1867, he was named to the Senate by the royal proclamation issued under the British North America Act, 1867.