Annette Wilcox, aged 62, rattled tins at crowd-pulling events like car boot sales and steam rallies up and down the country, but failed to give the cash to the charity.
Shrewsbury Crown Court heard that it was "even more heartbreaking" given that Wilcox had suffered from cancer herself in the past, as have some of her loved ones.
But it did not stop her taking from the charity, which provides specialist healthcare, information and financial support to people affected by cancer. She was collecting fraudulently between July 2015 and June 2016, and between September 2019 and May 2020.
The court heard that Wilcox approached the charity to become a fundraiser and was provided with promotional material, collection tins and a letter of authority.
Wilcox, who has two previous convictions for benefit fraud, attended events for Macmillan on at least 22 occasions, although Recorder Charles Thomas said in reality it was probably more.
Catherine Donnelly, prosecuting, said Wilcox would likely have collected between £500 and £1,000 from each event. Recorder Thomas said he could not be sure how much money Wilcox conned the charity out of, but that he could be confident it was at least £5,000.
Wilcox, of Arden Avenue, Dawley, Telford, pleaded guilty to two counts of fraud by false representation.
Her defence advocate said he did not believe public confidence would be eroded enough not to give to charity by "rotten apples" like Wilcox.
He said that Wilcox kept money to pay for pitch fees and tombola items, and that she had recently been diagnosed with early onset dementia.
Recorder Thomas jailed her for 12 months. He told Wilcox that her "lack of remorse" meant the "only appropriate punishment" was one of immediate imprisonment.
A tearful Wilcox wailed from the dock as she was taken down. She will serve half of her sentence in custody and will then spend 12 months on licence.