Johnson, in remarks to a business audience in Ottawa today, pointed to last year's Boston Marathon bombings as an example of terrorist threats that are difficult to predict.
Johnson also said cybersecurity is one of his top priorities and a common concern when he talks to his foreign counterparts, with the U.S. and U.S. companies facing attacks from a number of actors. He added that the "legal uncertainty" around information sharing needs to be resolved.
In his midday speech to the Canadian American Business Council, he also spoke about measures by the U.S. government to improve the flow of good across the border while maintaining security.
Afterwards, Johnson took part in a moderated question and answer session, and took questions from the audience and reporters.
Johnson arrived in Ottawa on Monday for a two-day visit that has included bilateral talks on counter-terrorism with several ministers of Stephen Harper's cabinet.
The meetings come as both Canada and the U.S. face intense pressure to clamp down on the recruitment, fundraising and organizational activities of ISIS in North America.
Officially, Johnson's office billed the trip as an opportunity to "discuss engagement through the Beyond the Border initiative."
That joint effort, intended to make legitimate trade and travel between Canada and the U.S. easier while still managing security threats, has experienced delays and complications.
Aside from Tuesday's speech, Johnson was not expected to make any formal announcements or other public statements while in Ottawa.