Mathilda Jones was the aunt of Jupiter Jones, co-owner of The Jones Salvage Yard, and Titus Jones' wife. She was described as a woman who loved to "put boys to work", whose "heart was as big as she was". She used an unique exclamation, "Mercy and goodness and sweetness and light," occasionally. She was described as a woman who would not miss any chance of earning money. This was evidenced when she became worried that the sleuths might drop one of the busts brought by her husband, that costed five dollars, and instructed them to leave the job to Hans and Konrad, who could carry without dropping.
In return for the work the three sleuths did at the salvage yard, she rewarded them with money, that they used for various purposes, and also in their investigations.
Aunt Mathilda also defended the boys against Cecil Percival when he threatened them with his cane while he had them trapped at the Jones Salvage Yard. She took his cane away from the larger man and flung it into a pile of junk.