In 2016, the U.S. Customs and Border Protection agency prevented 14,293 "high risk travelers" from boarding airplane flights. That compares with 2015 when the government's preclearance efforts denied boarding to more than 10,700 travelers.
The official said preclearance efforts are being strengthened domestically and overseas.
"We must continue on all these tracks to monitor and prevent terrorist travel," he said.
Johnson also singled out the FBI and others for "doing an excellent job detecting, investigating, preventing, and prosecuting terrorist plots in the homeland."
DHS, the newest and third-largest Cabinet-level department of the U.S. government, also sees public vigilance and reporting of suspicious activity as instrumental in preventing terrorism given what Johnson termed "this particular environment of homegrown violent extremism and the lone wolf actor."
Meantime, Johnson's exit memo also highlighted that cyberthreats are on the rise, and the need to do more to "fix our broken immigration system."
"As online threats grow, we must secure the Internet and the increasing number of Internet-connected devices and infrastructure," he said. "Cyber threats are increasing in their frequency, scale, sophistication, and severity. This affects everyone, across the country and around the globe."
According to Johnson, the department's National Cybersecurity and Communications Integration Center — the federal government's hub for cybersecurity sharing and incident response — sent out more than 6,000 bulletins last year and responded onsite to 32 cybersecurity incidents.
To further enhance cyber efforts, the federal government recently deployed an automated information platform to share cyberthreat knowledge with the private sector in real time.
Johnson also hailed U.S. cooperation with China on cybersecurity as an achievement of the Obama administration. He said the agreement between the two nations includes a pledge to "refrain from conducting or knowingly supporting cyber-enabled theft of intellectual property, including trade secrets, with the intent of providing competitive advantages to domestic companies."
Finally, the secretary said the issue of immigration remains a challenge.
"Spanning the Clinton, Bush, and Obama administrations, our government has invested more in border security than at any point in the history of this nation," Johnson wrote.
According to the DHS secretary, the Border Patrol agency "has never had a larger or more sophisticated inventory of technology and equipment to carry out its mission."
For example, the Border Patrol now uses drones, thermal imaging capabilities and underground sensor technology on the U.S.-Mexico border. The agency's number of unmanned aerial systems, or drones, has more than doubled in the past 16 years while the force of agents at the southern border also has roughly doubled during this time period.
"This investment has paid off," Johnson said. "Today, it is now much harder to cross our southern border without authorization and avoid detection and apprehension."
Johnson also said there's been "a dramatic shift in the demographic of this illegal migration that has presented challenges to our immigration system — fewer Mexicans and single adults are arriving, and more women and children, primarily from Central America, are fleeing poverty and violence and crossing our southern border."
He also defended the Obama administration's efforts "to fix the immigration system, make it more fair, and make our enforcement smarter and more efficient. The next administration and the next Congress must return to the subject of comprehensive immigration reform."