General Angus Campbell gave evidence at the final public hearing day for the Royal Commission into Defence and Veteran Suicide.
"Our people deserve and should rightly expect the support and care they need both during and after their service," he said.
"I acknowledge that this has not always been the case and tragically it's led to the deaths by suicide of some of our people.
"Defence is committed, and I am committed to doing better.
"I apologise unreservedly for these deficiencies."
General Campbell said the courage displayed by those who came forward to share their experiences was deeply admirable.
"I sincerely appreciate the efforts of those who have contributed to my learning and our deeper understanding of suicide and suicidality and its enduring aftermath," he said.
The defence force chief will be the sole witness to appear at the commission on Thursday, which will conclude in Sydney.
It comes after defence department secretary Greg Moriarty on Tuesday denied suicide among veterans was until recently a "blind spot" and said his understanding of the issue had changed.
Counsel assisting Erin Longbottom KC asked Mr Moriarty whether it would be fair to say the department had a "blind spot" on veteran suicide before the 2020 Afghanistan Inquiry, also known as the Brereton Report, and other probes.
"No, I wouldn't accept that," he replied.
"Those reports and our experience of recent years has intensified focus on these issues."
Mr Moriarty revealed that there are not enough mental health workers able to help defence members due a nationwide shortage of trained professionals.
The department spent $7.2 million on mental health and wellbeing in the past financial year but Mr Moriarty didn't believe a funding injection would address the issue.
The Royal Commission will be adjourned after Thursday's hearing until the Ceremonial Closing Sitting in Sydney on August 28.
Its final report is due to be handed down in September.
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