The Independent Lothians MSP said the defeat of her End of Life Assistance Bill by 85 votes to 16 with two abstentions was much as she had expected.
But she said she would bring forward similar legislation again if she remains an MSP after the election in May.
She said: "Parliament's will must be accepted, but parliament's will can change.
"If I'm elected next time, people will know that I'm going to pursue the idea and I'll be able to say there's some sort of mandate implicit in that."
Ahead of yesterday's debate, Ms MacDonald revealed poll findings showing nearly eight out of ten Scots backed her.
The Bill would have allowed people whose lives had become intolerable because of terminal illness or a degenerative condition to ask for help to end their lives. It included a number of safeguards, such as requiring registration with a medical practice for at least 18 months, a formal request to a doctor, a second opinion from a psychiatrist and then a 15-day cooling-off period before a second formal request.
Ms MacDonald, who suffers from Parkinson's disease, has said she would like the reassurance of knowing she could decide when her life should end if it became unbearable.
The committee set up to examine the Bill recommended it should not proceed any further through the parliamentary process. Committee convener Ross Finnie cited a catalogue of concerns, ranging from the effect on wider society of such a change in the law to the definition of "terminal illness" and differing understandings of a "dignified" death.
He told MSPs: "The committee found the ostensibly objective eligibility test based on finding life 'intolerable' to be inherently subjective."
Health Secretary Nicola Sturgeon praised Ms MacDonald for raising such an important and sensitive issue and creating the atmosphere in which it could be debated in a mature and sensitive way.
She said the Scottish Government's view was that the current law was clear - it was not lawful to assist someone to commit suicide; but decisions about prosecution were up to the Crown Office.
Lothians SNP MSP and former Wester Hailes GP Ian McKee urged MSPs to allow the Bill to be considered further.
He said while enormous progress had been made in the field of palliative care, there were some patients whom palliative care could not help.
"It is because such patients exist I support the principles of this Bill," he said.
But the SNP's Roseanna Cunningham spoke out against the Bill and suggested even its title was misleading. "Being helped to kill yourself is the very antithesis of end of life assistance."