She urged her successor, Rishi Sunak, to be “bold” in order to boost the economy in an unapologetic farewell speech.
She said the UK “cannot afford” for government spending to take up an “increasing share of our national wealth”.
And she insisted “brighter days lie ahead” for the UK.
Flanked by her husband and children, Ms Truss said she would go back to being a backbench MP for her South West Norfolk constituency.
Mr Sunak will take power later after he is formally appointed by the King following his victory in the Conservative leadership contest.
Ms Truss announced her resignation last week after just 45 days in office, after big tax cuts in a hastily-assembled mini-budget prompted financial turmoil.
After initially defending her agenda, she later abandoned almost all of it in a bid to calm markets but saw support from her own MPs ebb away.
But she struck a defiant tone in her speech in Downing Street, adding: “I’m more convinced than ever that we need to be bold and confront the challenges we face.
“We simply cannot afford to be a low-growth country where the government takes up an increasing share of our national wealth,” she added.
Quoting the Roman philosopher Seneca, she added: “it’s not because things are difficult that we do not dare, it’s because we do not dare that they are difficult.”
She also pointed to her reversal of April’s National Insurance rise and her plan to limit energy rises as examples of successes during her short time in office.
She also urged Mr Sunak to continue British support for Ukraine, adding the country “must prevail” in its “brave fight against [Vladimir] Putin’s aggression”.