A former Calgary mayor credited for protecting many of Calgary's park spaces died in his sleep Monday at the age of 90.

Jack Leslie held the city's top job from 1965 to 1969 and was the first native Calgarian to hold that position. He served one term as an alderman before becoming mayor.

His best known legacy is the preservation of Calgary's parks and protection of its riverbanks.

He was one of the only politicians to protest a plan by CP Rail in the 1960s to move its tracks to the shores of the Bow River, which others said would ease traffic congestion caused by the railway.

Leslie fought the plan by saying it would be a grave mistake for future generations.

Leslie, a former air force pilot, had been set to enter the family real estate business after he and his wife were married.

But he ended up entering the world of politics instead when he ran for alderman in 1961 and won.

The person currently holding Leslie's old job says Leslie made green spaces matter to city hall.

"I think the legacy that Jack Leslie left, maybe not totally for him but the path that he started us on, was the most impressive one and one we should all be very grateful for," said Coun. Gael MacLeod.

After four years as mayor, Leslie lost the chair in 1969 and returned to the private sector, joining the real estate business his father had started half a century earlier.

There is now an annual environmental award given to high school students in his name.

Leslie is survived by his wife, their three daughters, and grandchildren.