What was the first professional league of lacrosse?

In 1974-1975 was the original National Lacrosse League (NLL) which has no connection with the league of that name which exists today.

Then the Canadian University Field Lacrosse Association (CUFLA) started in 1985.

In 1987 there was the Eagle Pro Box Lacrosse League. It changed its name to the Major Indoor Lacrosse League, then later to the National Lacrosse League (NLL).

Then Major League Lacrosse (MLL) started in 2001.