He also said he contacted The Associated Press, which counts votes, and its CEO, Gary Pruitt.
"Prior to Election Day, I also personally reviewed with the CEO of The Associated Press its long-standing election-day reporting process, including the redundancies and safeguards in its systems," Johnson said.
And while Johnson said Russia did not "through any cyber intrusion alter ballots, ballot counts or reporting of election results," he said he was "not in a position to know whether the successful Russian government-directed hacks of the DNC and elsewhere did in fact alter public opinion and thereby alter the outcome of the presidential election."
Johnson also said he was not happy that he learned well after the fact that the DNC's computer systems had been hacked. He said he became aware of the compromise "sometime in 2016," and that when he pressed his staff on whether the DHS had been sufficiently proactive to help identify the intruders and patch vulnerabilities, the answer wasn't reassuring.
"The FBI and the DNC had been in contact with each other months before about the intrusion, and the DNC did not feel it needed" Homeland Security's assistance at that time.
He also said he wasn't aware that the FBI had opened a counterintelligence investigation into possible collusion between the Trump campaign and Russian officials. But he said former FBI Director James Comey would not have undertaken such a probe lightly and without a basis for doing so.
Johnson was homeland security chief for the Democratic president from December 2013 to January 2017.
The Senate committee was hearing from officials at DHS and the FBI's counterintelligence division. Special counsel Robert Mueller is conducting an inquiry into possible collusion between the Trump campaign and Russian officials.
Trump has decried the investigations as witch hunts and has rejected the assessment by U.S. intelligence agencies that Russia's hacking and disinformation campaign was intended to aid his candidacy.
Johnson's designation of U.S. election systems as critical infrastructure was aimed at providing more federal cybersecurity assistance to state and local governments.
Johnson announced the shift on the same day as the release of a declassified U.S. intelligence report that said Putin "ordered" an influence campaign in 2016 aimed at the U.S. presidential election. That report said Russian intelligence services had "obtained and maintained access to elements of multiple U.S. state or local electoral boards."
None of the systems targeted or compromised was involved in vote tallying, the report said, and there was no indication Russia's prying changed vote counts in key states.
But Johnson's decision triggered an outcry from state and federal election organization officials. They complained that Johnson's department failed to respond to questions and concerns they had about the designation before the change was made.
American elections are highly decentralized. Voters cast ballots in roughly 185,000 precincts spread over 9,000 jurisdictions during the 2016 presidential election. Elections are also subject to rigorous and elaborate rules that govern how and what equipment is used.