Ultrasound-assisted extraction is an interesting tool for obtaining bioactive compounds from plant matrices applicable as agricultural bio-inputs, as it increases the extraction efficiency, reducing the process time and the use of solvents. This technique uses ultrasonic waves to break down plant cell walls, releasing bioactive compounds quickly and effectively and promoting a sustainable path to obtaining bio-inputs. Accordingly, this research study reports pioneering results regarding the herbicidal and fungicidal potential of different extracts obtained from Senecio brasiliensis (samples from flowers, leaves, and stalks) and Solanum viarum (samples from fruits and roots), two weeds typically found in rural areas of South America. The fungicidal activity of the samples was tested on two fungi, i.e., Fusarium graminearum and Sclerotinia sclerotiorum, while the herbicidal action of the extracts was evaluated in pre-emergence tests in cucumber (Cucumis sativus) seeds. The successful results indicated a high antifungal and herbicidal potential of the extracts obtained for both weeds, with the inhibitory effect against both fungi achieving up to 82%, and the inhibition of C. sativus seed germination reaching 100% for all samples.