Well before the coming of the first European settlers, Canada's aboriginal peoples had discovered the food properties of maple sap, which they gathered every spring. According to many historians, the maple leaf began to serve as a Canadian symbol as early as 1700. In 1834, the first St. Jean Baptiste Society in North America made the maple leaf its emblem. In 1836, Le Canadien, a newspaper published in Lower Canada, referred to it as a suitable emblem for Canada. In 1848, the Toronto literary annual The Maple Leaf referred to it as the chosen emblem of Canada. By 1860, the maple leaf was incorporated into the badge of the 100th Regiment (Royal Canadians) and was used extensively in decorations for the visit of the Prince of Wales that year. Alexander Muir wrote The Maple Leaf Forever as Canada's confederation song in 1867; it was regarded as the national song for several decades. The coats of arms created the next year for Ontario and Quebec both included the maple leaf. The maple leaf today appears on the penny. However, between 1876 and 1901, it appeared on all Canadian coins. The modern one-cent piece has two maple leaves on a common twig, a design that has gone almost unchanged since 1937. During the First World War, the maple leaf was included in the badge of the Canadian Expeditionary Force. Since 1921, the Royal Arms of Canada have included three maple leaves as a distinctive Canadian emblem. With the proclamation of Canada's new flag in 1965, the maple leaf has become the most-prominent Canadian symbol. In 1939, at the time of World War II, many Canadian troops used the maple leaf as a distinctive sign, displaying it on regimental badges and Canadian army and naval equipment. In 1957, the colour of the maple leaves on the arms of Canada was changed from green to red, one of Canada's official colours. On February 15, 1965, the red maple leaf flag was inaugurated as the National Flag of Canada. http://www.pch.gc.ca/progs/cpsc-ccsp/sc-cs/o3_e.cfm

Short answer as to how it came to the current flag: In World War 1, Lester Pearson noted that almost every battalion from Canada included the maple leaf in its insignia, and vowed he would campaign to put it on the flag, and of course 50 years later as prime minister of Canada he was part of the 33-day debate that resulted in the maple leaf as the Canadian flag. Stolen verbatim from :http://fotw.vexillum.com/flags/ca.html#mpl

The maple tree.